I did something similar to this at one time. I have a truck. Installed a keyless entry system on it. It was the generic kind made for cars, vans, and trucks. So when it was all installed, I had one function left over.... the trunk popper. What I ended up doing was using the relay contacts for the trunk output to short out the button on the remote garage door opener permenantly mounted in my truck. Now I can start the truck and open the garage door from the same remote. Ofcourse in my case it was kind of a remote controlling a remote control, which is not what your original plan was, but an idea to play around with.