I have two keys for my 2000 Lincoln Navigator, and two fobs, both programmed to an aftermarket car alarm system my brother had the dealer install when he bought the car. I wanted another key, which means, of course, another fob. I got the key online for $10, had the hardware store cut it and programmed it myself. Easy.
I then bought a fob (took the FCC number on the back of one to find the right one) and ordered it. Tried to program it, and had no luck. The video below explains it fully. Any suggestions would be appreciated.