I bought a homelink transmitter from eBay. While it didn't work and I have to get another one, I did take some pictures for a DIY write up.

TOOLS REQUIRED:
 - Fingers
 - Sharp Knife
 - Safety Goggles
 - Gloves
 - Wire Cutters
 - Needle Nose Pliers or Small Screw Driver

RECOMMENDED:
 - Volt Meter

WARNING: This requires tapping into constant hot 12v and cutting your head liner. Your results may vary. I'm not responsible for the outcome.

Before starting this I recommend testing your used homelink system. Program it like this:

1. Press the two outer buttons for 20 seconds until it flashes rapidly.
2. Unplug your garage door.
3. Holding the garage transmitter 4 to 6 inches away, press the button to send the signal and press the homelink button you want assigned to the door.
4. Release the both buttons when the homelink light changes from flashing slowly to rapidly.
5. Plug the garage door opener back in and try it. If it doesn't work go to step 6
6. Find the button on your garage door opener that enables it to learn a new remote. Press the button and within 30 seconds press the homelink button for the door. (You may have to read the garage door opener manual to figure this one out.)

INSTALLATION:
1. Find where you want to put it and mark the hole. Cut it with sharp utility knife.


2. Pull on the front of the overhead light assembly to take it down.


3. Remove the wire stubs from the Homeline transmitter.


4. Using a small screw driver or needlenose pliers, push the new wires into the Homelink wiring connector.


5. Run the wire to the light assembly hole.


6. Cut the sheath back from the wires for the light assembly. Attach the Homelink wires using wire taps. NOTE: The wires on the light connector are the same color. Use a volt meter to verify the +12v and Ground.



7. Gently but firmly shove the Homelink transmitter into hole. You will have to move it around a bit to seat it properly. You may have to remove the tabs on the back side to get it into place.


There you go. Good luck and happy garage door opening.

sorry it didn't work but very cool install and GREAT install... i applaud you!

I wonder if there's a way to build it into the overhead console...

Did mine this weekend.  Picked it up on ebay from an '07 Caliber for $22 shipped!  Put mine where if I get the overhead console, I can mount the homelink somewhere on the console.




Here's my wiring...shown taped up with wire taps and soldered wire extension with heat shrink tubing.  Use test light to find + wire from map lights.


There is a foam block that I had to gouge into to make room for the unit, but there is enough room for my hand AFTER I removed the sunvisor clips(twist to unlock them).

Then pry tabs back on base and remove.

There is the 2" square block that is glued to the headliner.  I just stabbed at the base of it a few times with a screw driver to losen it, then dug out what I needed to fit the homelink box.



FINISHED!!!  Go to http://www.homelink.com/home/home.taf to program homelink with your garage door opener.

Great write-up and great pictures. Left Karma for you.
I really wanted to do this mod but the used homelink unit I bought off ebay did not jive with my garage door opener so I did this instead:
http://newscionxb.com/do_it_yourself/garage_door_opener_remote_button-t207.0.html