Remote Entry Programming

I have a 1999 Taurus Wagon with remote entry and have two remotes already programmed for it. I have a third remote I tried to program with the other two, but it won't accept it. Is there a limit of remotes you can program at one time. I know with my Marquis I could program up to 4 remotes. Anyway, when I go thru the programming routine, it accepts the original two that came with the car, but won't accept the third new one. See below for the Fob #s for all three. Thanks for any help you can give me. I want to velcro the third one just inside the rear tailgate so when I take packages out I can just reach over and push the lock button. It is a bugger to reach into the pocket and fish around for the remote when you have your hands full of bags etc.

Ok, here are the Fob #s:
3B-F87B-15K601-AA This one works
F78B-15K601-BA This one works
F8AB-15K601-AA This one doesn't work

The numbers don't matter for the Taurus, as long as they are black, and not gray.

If they are all black, you probably just have a battery worn out in the nonprogramming unit.

Where can you get programming instructions? How do you program the keypads.
I have a 1994 mustang, and need to program a keypad.

f8ab-15k601-aa
FCC ID : CWTWB1U313

will it work?