Very cool and effective cheap DIY. Added the radioshack switch into the ashtray as OP. I soldered the wires on to the existing opener, cut into its plastic to get the wires out and snapped back the opener cover just like it was before. There is plenty of room for it to sit by the shifter area. It is also easy to replace the battery although I installed a new one anyway so it should be good for the many years.
One problem was that every time I would snap the cover together, my door would start opening and closing randomly. Turns out that the wires were stripped just enough that when you snapped the cover closed they would touch the board at random spots and cause this crazy behavior. A little tape around the two wires next to the soldered points solved that.
The other issue was that I picked up that grommet mentioned in this thread. I cut it into two halves so basically just had a thin rubber washer between the switch and the ashtray. It looked great, i installed it and was very happy until i closed the ashtray lid and the door started to close again. Apparently, the washer (half of the grommet) was just thick enough to raise the switch where it would be triggered every time the lid was closed. Annoying.... Cut that thing off and went back to the OP design and all is well. Not sure what i did wrong with that grommet or maybe the clearances are slightly different in models, but it did not work for me. Who cares though - it looks great without it anyway.
Like everyone else said - very cool and satisfying mod. I was looking for the homelink but don't feel like paying 70+ when i can have the same functionality for a lot less.