Well I feel slightly dumb. Trying to leave work... Not starting. No fuel pump prime.... Theft light blocking rapidly... First time ever noticed it!!!! Got another key on the way hoping that's all it is? Anyone on with anything I can do or what all I might be getting into with this issue if it's not my key? Urgent timely answer would be lovely lol
Edit... Second key does same thing. Rapid flashing of theft light. So basically sometimes this has worked something not over last few weeks. But this first time I've actually looked for and noticed the theft light
Rapid flashing Theft Light after turning key to RUN means PATS(passive anti-theft system) could not "read" the key's number, or it was a non-matching number.
PATS Keys have an RFID Tag embedded in the handle, each tag has a unique number, RFID tags require no batteries.
When you turn on the key a ring around the key slot "reads" that tag number and sends it to the PATS module, this could be separate module or part of the main computer(PCM), 1999 has separate PATS module.
At the factory the PATS key's tag numbers that come with the new vehicle are programmed into PATS memory.
A mobile locksmith with Ford software can added new PATS keys to the system, or a Ford Dealer.
So either your keys RFID tags have been damaged, or the Antenna(ring) around the key slot is damaged, or PATS is faulty.
One thing, make sure you do not have 2 PATS keys on the same key ring, even if they are for different vehicles.
Although rare, if 2 PATS keys are in range of antenna, when it reads the number it gets a garbled number from the 2 keys, causing intermittent issues.
PATS keys can also be tested, to make sure the number that comes out is not "garbled".
Since you have 2 factory keys and neither work full time it points at antenna or module issue.
Antenna is also called the transceiver.
You can try a reset by unhooking the battery for more than 10 minutes, but long shot at best