I have mine velcroed to the underside of the left glove box lid and a pair of wires soldered across one of the switches. The wires are then run to a waterproof momentary push button switch mounted on the same glove box lid. Switch was obtained from China via ebay for $1.90 shipped. I got an extra opener on ebay a couple years ago for around $10 shipped.
BTW - not all remotes are 12v. Mine uses a 3v watch battery.
I did the same thing. But I used a non-waterproof switch from radio shack that I need to replace about once a season. I tried to find a waterproof one with no success. I'll have try searching again.
Left front jacket pocket, I just hit it through the jacket with my gloves on as I roll up the driveway...