Add keyfob keyless entry

[tai]

I bought a 2003 Explorer that did not come with the keyfob to lock/unlock doors. I was told by dealer that buying a keyfob and programming it was all that was needed as long as the car had electric locks (it does). Bought one on line and used programming instructions with no luck. The car does not respond by clicking the door locks after 8 on-off cycles as detailed in the instructions and does not respond to the keyfob. Has anyone added the keyless fobs to a vehicle that did not come with them. Is there another modification necessary to enable them to work?

1/30 update: After contacting a different Ford dealer, the modification is apparently extensive. I was quoted $297 parts and $308 installation, so an aftermarket system is the way I will be going, should be less than $200 total.