Did this mod not long ago, hooked my Garage Door Opener (GDO) to Passing Light switch using existing switch wairing. Only downside of doing it this way is GDO battery drain while using High Beams (unless you have 12V unit).
Since my next mod will be HID Kit, I'm not worried about riding on High Beams no more.
You will need Remote Garage Door Opener (i purchased mine a while ago, finally found good use for it).
Pop the cover off your remote control unit and locate the button you would like to control. You could do multiple buttons if you needed to, however you'd need a different switch than what I used. You should be able to figure out where to solder the wires to by looking at the greenboard.
Since I decided to use my Passing Light button, I needed to get small 12v relay switch to controll my GDO.
Switch has 5 connectors, simply connect power source cable + and - to coil terminals and wire your remote opener in "always closed" position according to the relay diagram.
I used left top row (negative) and middle 2nd row (pass light switch) connectors to wire my opener.
Mounted under top left cover with 2sticky side tape. All left to do is push that yellow button.