diagnostic procedure for key fob

[roberttlove]

Howdy,

1995 Explorer XLT with keyless entry door keypad bought used without
any fobs. Research indicated that since I have the keypad on the
door, then the factory also supplied keyless entry remotes. I picked
one up on eBay that claims it is a working specimen: Transmitter 1,
FCC ID: GQ43VT4T, DOC: 1470 K763, TRW Inc 3165189. It's a three
button (lock, unlock, panic) labeled "driver door only".

I replaced the two batteries in the remote. I've cycled the ignition
switch 5 times with either OFF->ON or OFF->ACC (get the "entering
programming mode" lock/unlock) and then pushed any combination of
buttons on the remote, but I get no confirmatory lock/unlock from the
vehicle.

Can someone help me with diagnosis? I think it's most likely the fob,
but before I go back to the seller, I'd like to have some evidence or
diagnostic logic that points the blame away from the vehicle
electronics.

Anything procedural-wise I'm doing wrong?

Thanks for your help,

Robert

(btw, also posted to rec.autos.makers.ford.explorer)