I did something similar to my 03 Jetta about a year ago.
1) Take your existing garage door remote.
2) Solder the switch contacts closed. (It's like making the remote on all the time)
3) Instead of a battery, tap into the voltage wire for the high beams and add a regulator to step it down to 9 volts.
4) use this 9 volts for the battery replacement in the remote.
Here is what happens. You flash your high beams, 12 - 14 volts goes through the regulator (Which you can built for about 5 bucks from radio shack) and energizes the remote with 9 volts. Switch is jumpered from step 2, causing the remote to transmit signal and the door opens, or closes.
I mounted mine under the hood and it has worked great.
Tom
