What does it mean when my 2003 thunderbird anti-theft light is flashing?

SOURCE: 2000 ford windstar anti theft light flashing vehicle won't start

This means that your key is not being recogonized.Ethier the chip in the key is bad or the patts transciever at the lock cylinder. Do you have a spare key? If not you will have to get 2 new keyss cut and programmed.

SOURCE: 2003 Ford Windstar won't start

does it have an autostart? if you have two keys (this will only work if you have two) turn the key to on in the ignition and wait till the theft light goes out do this for each key this may fix your problem.

SOURCE: 98 explorer will turn over but not start

Yes you have a problem with the theft system. Have you tried a different key? It has a transponder system.

SOURCE: ANTI THEFT LIGHT KEEPS FLASHING ON AND OFF

Did you steal it? I heard about you thieves who steal cars then put new transmissions in!!
Seriously, try this, it may help...
Put the key in the ignition, and turn forward all the way just before cranking. Then within 5 seconds, locate, press and hold in the valet/overide button, found under the dash area, (push button, spring loaded) until the installed LED (the blinking light) in your dash area goes on solid, and the siren chirps once. (Your valet switch might be the toggle (on/off) type. in this case, key on, flip switch in opposite direction, then turn the key off.) This should reset the alarm, and reset the grounded relay on the starter wire, allowing you to start the engine. At this point, the only thing working would be the keyless entry from the alarm, if it came equipped that way. Just Repeat these steps to place it back to usable armed mode. You may get 2 chirps, confirming it, and the lights may flash twice as well. That should do it. Check out www.explorerforum.com

SOURCE: The car wont start, the

TRY REPROGRAM KEY .PUT KEY IN IGNITION SWITCH TURN IGNITION SWITCH ON POSITION 20 SECONDS DONT START ENGINE WAIT 20 SECONDS TURN OFF SWITCH THEN TURN SWITCH BACK ON 20 SECONDS WATCH THE THEFT SECURITY LIGHT BLINKING.WHEN THEFT STOP BLINKING VECHICLE WILL CRANK.IF LIGHT STILL BLINKING.TRY FEW MORE TIMES KEY IN SWITCH ON POSITION FOR 20 SECONDS.MAKE SURE YOU HAVE THE CORRECT CAR KEYS IN SWITCH IF NOT WRONG SET OF CAR KEYS WILL SET OFF THEFT SECURITY SYSTEM.IF KEY RESETTING TRICK DONT WORK.CHECK BATTERY POWER MAKE SURE YOUR BATTERY HAS 12.5 VOLTS AND CHECK SECURITY THEFT FUSE.IF ALL LOOKS GOOD YOUR SECURITY HAS TO BE CHECKED OUT AT THE DEALERSHIP.