Starter will not disengage, whats wrong - 1988 Ford Aerostar

It is ver important that the correct bolt spacers between the starter and the mount be used to ensure the correct engagement - you either have the wrong ones or no spacers installed

the wires at the ignition switch are wrong
the power to the relay should only be hot when the key is in the start position and not the run position

If it is recently installed could be wires wrong on the starter, If it is old starter just acting up could be a sticking solenoid or starter drive on the shaft

Replace the ignition switch

The teeth on the flywheel or starter drive could be damaged enough to cause sticking and it does not release. Another reason could be the clearance between teeth on the flywheel and starter drive are too tight. The starter would need more metal tabs or spacers to increase the clearance distance between the teeth on each one.

Sounds to me like your starter did not disengage and was running the whole time when vehicle running. From your key to the starter is a "trigger wire" which activates the starter when you turn the key. If that wire end touches other wires on the starter then the solenoid will stay on keeping the starter running which will then over heat and burn out the starter. If vehicle was running fine before the starter replacement there is really nothing else it could be.

wrong starter. it is too long

sounds like soleniod.pull starter & bench test it.lets hope ignition switch is OK.

smaller of the 2 wires is on the wrong terminal,or the starter needs a shim to move it away from the ring gear,if it is too close it won't come out.

I think you have the wrong starter on your vehicle... remove it before your flywheel/electrical system is damaged! Take it back to the place you got it and ask them to help you resolve it.