Starter won't disengage even with key out - 1991 Ford Mustang

Try new starter or even try bridging a screwdriver to engage the starter manually that will tell u if its starter or something else

Replace throw out bearing and spring

replace starter solinoid before u catch ur car or fire

Bad starter or the solenoid on the starter replace before you damage the teeth on the flywheel

There is a spring which may be weakened or broken that retracts the gear. Only a service will fix.

Find the starter relay under the hood. It may be bad and stuck so power continues going to the starter. Try a new relay Or try pulling off the small signal wire on the relay (it is from the switch), and hook battery up. If starter continues to run, the relay is bad. If starter is stopped, put the signal wire back on relay. If starter now starts operating, and key is still off, then switch may be bad .Location of relay may be at left front fender apron, in front of shock tower (like on earlier taurus models).

more then likely its a bad starter. or the cable to starter.

Sounds like your starter solenoid is stuck in the on position if you can get down there and have some one hook the battery on, hit the starter with a hammer some time this will disengage the starter.

sounds like to me that the ignition is bad and sticking on the start positon

You have a defective start, no 2 ways about it, and it may be hung up, which means there is no way that you will be able to start it or to disengage the starter. It will have to be towed, and as it is, the tow company can tow it.
jetski