I've installed a few of these in my day, and it is difficult to make it anything less than a hack-job. There's a fist-sized bundle of wires, and each of them have to be spliced into something. On the newer cars you'll have to bypass the RFID chip in the key, as well. Either by securing an extra key under the dash, or using a module to bypass the system electronically. I've seen both done. I doubt there's much of a theft-risk on these Town Cars, so I'm not too worried about the end effect of bypassing the security.
Interestingly enough, this Town Car I just bought appears to have one installed, but I was not provided with a remote. I have yet to dig under the dash to find out just how shoddy the install is.
If you are doing it yourself, and don't mind sitting upside-down in your driver's seat for a few hours, it can be done well. It is something I wouldn't trust most shops to do a good job on though.
Edit: I wonder if I just figured out why the previous owners only had one key to give me with the car...