Here is a little walkthrough of disassembling the the fridge and modifications that I made. I'm using a Frigidaire FFPH44M4LB fridge. It's 4.4 cu. ft. and will fit 2 Cornelius kegs and a 5lb C02 tank in the back.
A couple months ago I decided I'd had enough of drinking stale, bottled beer and that the solution to this problem would be to build a kegerator and get the brewery to fill my keg with fresh brew. Of course, later on I found out that none of the breweries will actually fill personal kegs, at least here in Chicago. C'est la vie. I'll be brewing my own batch soon to fill it instead.
Gather your parts
Disassemble the fridge
My fridge had a freezer/icebox in the top of it. It popped right out by removing a couple screws on the door and sliding out the shelf. I knew that since it had a freezer in the top, it probably had a gas line above it. If you drill and you cut the line you will ruin the fridge. Better safe than sorry.
Remove the lid
This is probably the hardest part. It's glued down tight to the foam insulation and the sides of the fridge. Use a pry tool to slowly work your way around and remove it (You'll want to glue it back on after you're finished).
Find the line
I found this thread on Home Brew Talk to be extremely helpful. I found where the safe places to mount the tap probably were (in the front), but I wanted mine in the back. It was necessary to cut the insulation out to find the line (carefully, of course) so I could drill around it. I used a spatula, Leatherman, and a bicycle tire iron to remove the insulation.
Drill your holes for the tap
You'll want to line up everything so you can drill through to mount the tower. Run your lines through and put some excess insulation back in. I mounted a small piece of wood under the lid to help stabilize the tower. Remember, measure, and then measure again. And then do it again just for good measure.
Cleanup, and fill it
Once you fill the keg you'll want to let it sit for close to 24 hours while it settles or else you'll lose a lot of beer in foam, and no one likes that. You'll probably need to tweak a bit a first to get it flowing nice. I keep a little thermometer in a cup of water to measure the temp.
Keg pressure: 10-12 psi Temperature: 38 degrees