Seeing as how the fuse block works properly with everything else until you hook up the Grip wiring I doubt the problem lies there. You say that when you hook up the grips to the battery it seems to work fin but my guess is that it doesn't.So that leaves four things as the potential problem, switch, relay, wiring, grips. As Rick mentions, the wiring would include your ground so check it out. Here is another thing you can do. My understanding is that these grips draw ~3 amps on high, but whether it is 3 or 5 amps, you can always but a meter inline to see what the draw is.
Lots of ways to handle this. I would say to first verify all wiring, then check the switch (I read where somebody was actually having problems with their switch), then you can check the function of the relay with your meter.