What is wrong with my 1991 aerostar the wipers won't turn off unless I turn the engine off?

The issue with your 1991 Aerostar's wipers not turning off could be due to a faulty wiper relay or switch. It's also possible that there is a short circuit in the wiring or a problem with the wiper motor itself. To diagnose the problem, consider checking the wiper control switch and examining the relay for any signs of damage. If you're unable to resolve it, consulting a mechanic may be necessary for further troubleshooting.