I've done this addition to all my bikes. An easy way to open the garage door.
I ordered this Skylink remote on Ebay but I've also seen them at Home Depot. It's small so it's easy to hide.
Start by dis assembling the remote by removing the screw on the back of the remote. You'll have to cut a notch in the side of the case to pass the wires thru.
These two solder joints on the back of the circuit board is what you're looking for.
I used solid copper wire and crimped two blade connectors to the ends. It really doesn't matter which wire goes to which side of the switch.
Re assemble the unit and hit the notch with some RTV to seal it up.
I mounted mine where the tool kit used to sit . Right next to the amp for my speakers.
The switch I chose is a normally off push button switch that I found on Ebay. I mounted it under the seat next to the volume control for my speakers. The bracket is mounted with double sided tape.
It really is an easy project. It will take all of an hour to put together.