Theft light flashes real fast on 1999 Ford Mustang

car cranks but will not start

Sounds like a PATS(passive anti theft system) problem...turn your key on then off quickly 10 times in a row, (on the 8th time you should hear your automatic door locks engage) and then after the 10th time it should start.
2 more answers
I had same issue. The anti-theft thinks you are stealing the car. Sometimes it will crank but only if the light is flashing at a regular pace but if it is too fast then it will not turn over. Have a mechanic put a new ignition harness on it.
How many keys do you have? Do they all have the same response? If so , they're telling you to get another car (toyota)