This was a known issue when I bought this '91 awhile ago...the guy takes the seat off to show me how easy it is and what kind of decent storage space there is, and there's a box of 40 or so 10-amp pink fuses there....he say's "Oh yeah, there's a little fuse problem, every once in awhile you will lose all the lights...." and he proceeds to show me where the fuse is...turns out it's the "Tail" fuse in the J-Box, and covers the tail light, directionals and dash lights, probably something else too that I'm missing...so has anybody else had a prob with this fuse, and managed to track down the culprit?
So far it's only popped on me twice, but tonight on the ride home I got near triple digits toying with a Harley 1200 Sporty, and damned if the lights didn't disappear as I was getting off the freeway.....not the headlight, thank God, just the peripheral lights and the tail...and then damned if the new fuse didn't pop just after I re-started the bike after putting the screwdriver away and putting the seat back on...
Anybody got any ideas?

Sounds like a short to ground somewhere.  Duh right?  Test light and a cold adult beverage of your choice and start digging.  Also, someone else will chime in here soon, but I'm wondering too if there isn't an issue in the junction box with a short.  That's really my first thought. 

I had the same exact problem.  Mine was a pinched wire in a harness near the forks, in the fairing.  It was getting pinched between the forks and the fork locks when turned to full lock, and severed one of the wires.  Then, whenever I turned the bars just so, the wire would touch the fork and short out that fuse.   

Put me down for a wire insulation damaged by rubbing the frame somewhere along the harness.
How to find it?  If it was I.... I would  inspect every place that might rub..without moving the wiring too much. You don't want to lose the fault area. If no luck seeing the problem, I would think about removing the wire off the fuse block or cutting it, or simply removimg the fuse and connect a 12v audible  (home made)  to hear while I was moving wires around.
 Also, you could remove the tank, put a slightly smaller fuse in, turn lights on and start moving wires.

12-volt audible...that sounds like it might have legs...can you give me an idea in layman's language on what that would entail? I'm assuming it would mean hooking up some kind of squawker to the blade clamps so that if a ground happens a chirp or squawk would ring out?

Yeah, pretty much it. In place of a fuse, connect something that runs off twelve volts and makes a noise or you could use a light, but the noise is better. I had voltmeter that had one built in. You can get a buzzer at radio shack. Connect some wire to it and stick the wires in the fuse socket in place of the fuse. Depending on the buzzer, you might have to observe polarity. The buzzer will buzz because the lights and whatever is on the circuit allows current to flow, but when the wires short to ground the buzzer will get the full 12v and you will hear the difference when you move the wires.

Depending on how much load is on the circuit and how sensitive the buzzer is to voltage changes, you might have to take some bulbs out. That way there would be less currant flowing through the buzzer when the short is not present.

Yeah, pretty much it. In place of a fuse, connect something that runs off twelve volts and makes a noise or you could use a light, but the noise is better. I had voltmeter that had one built in. You can get a buzzer at radio shack. Connect some wire to it and stick the wires in the fuse socket in place of the fuse. Depending on the buzzer, you might have to observe polarity. The buzzer will buzz because the lights and whatever is on the circuit allows current to flow, but when the wires short to ground the buzzer will get the full 12v and you will hear the difference when you move the wires.

Depending on how much load is on the circuit and how sensitive the buzzer is to voltage changes, you might have to take some bulbs out. That way there would be less currant flowing through the buzzer when the short is not present.

Make sure you have 1157 bulbs for the brake lights.  I switched to 2357 to have brighter brake elements and it kept popping the fuse..

Thanks, I'll check them tomorrow afternoon...

Make sure you have 1157 bulbs for the brake lights.  I switched to 2357 to have brighter brake elements and it kept popping the fuse..

I converted my rear directionals to marker/directionals with 1157 sockets and 2357 bulbs with no such problems, but that is only for the directional and marker circuits, I wonder why the brake circut won't handle it?

As to the fuse blowing, I'm with the short to ground crowd. My wife's Suzuki S50 had a similar problem with intermittant right front signal. The short version of the story is that it was a pulled/stretched wire in the harness right at the steering head.

Even a badly corroded light socket can cause resistive heat  which in turn makes a fuse go pop; also check for pinched wires shorting to ground. Remove bulbs and take meter reading of the (+) supply wires going to the sockets to see if you've got resistive readings to ground.

Correct me if I'm wrong, but do those wires run inside the rear fender? If so that's a prime spot to look. Sometimes with a heavy load you hit a bump and the tire hits the wire harness or just from vibration they get chafed.

you hit a bump and the tire hits the wire harness

That would be a BIG bump . . . like a MotoX bump . . . 

Thanks, I'll check them tomorrow afternoon...

Any luck? The first step is a visal inspection. I think doing a visal here might mean moving wires around, which might cause you to lose the short, so I think having the wire disconnected from the circuit on both ends (requires removing lamps and off the fuse.) connect an audible device and you inpsect and handle the harness.
 Next I might even try a meggar test     http://wiki.answers.com/Q/What_is_a_megger_test   There is no computer on the bike, so I thik you won't hurt anything.

That would be a BIG bump . . . like a MotoX bump . . . 

Actually, I haven't had enough dinero to get my shock replaced or upgraded in any way, so with my weight plus any passenger at all and we are bottoming-out HARD on the bigger bumps.....

Any luck? The first step is a visal inspection. I think doing a visal here might mean moving wires around, which might cause you to lose the short, so I think having the wire disconnected from the circuit on both ends (requires removing lamps and off the fuse.) connect an audible device and you inpsect and handle the harness.
 Next I might even try a meggar test     http://wiki.answers.com/Q/What_is_a_megger_test   There is no computer on the bike, so I thik you won't hurt anything.

Raining pretty good in these parts right now, bike is under cover and I don't have a dry spot to work on it for now...going over to The Home Depot later today to grab a volt meter, any suggestions for somebody with $40 or less to spend?

Yep. I have lots of opinions for free. Go to Harbor Freight and buy a $5 VOM and a $5 test light. They will very likely do anything you need them to. Alternatively, I don't have an opinion. Unless you're making money with it or using it every day, a cheap VOM will do fine.  I mean, this isn't an Italian sports car here.

Well, been through the entire system, pretty much...my mechanic is at the point where large steps need to be taken, he's thinking...so I've grabbed a complete wiring harness from the same generation off Ebay, and he's going to replace the whole thing, hopefully it's a bad insulation problem deep inside the harness...we DID find out why the directional sigs were having issues, the white wire had been worn off the back of the ignition  by incorrectly routed cables that rubbed the back of the ignition every time the bars were turned...that's all better, and I'm gratefull, as I use my signals pretty-much religeously...

Even a badly corroded light socket can cause resistive heat  which in turn makes a fuse go pop; also check for pinched wires shorting to ground. Remove bulbs and take meter reading of the (+) supply wires going to the sockets to see if you've got resistive readings to ground.

just read T's response and will add, I have seen numerous Conni's with corroded rear signal lamp sockets, that were realllllly ugly. I had one on my 86, that I almost lost a thumb over when the bulb popped trying to remove it...(I actually had a glove sitting there and didn't put it on....doh).
anyways, might check them both out also....