If they always park in the normal position when they stop, the motor and "park" circuit are okay. That leaves the 12 volt feed circuit and wiper switch. If they stop randomly anywhere on the glass the most likely cause is a bad ground on the wiper motor. The "run" and "park" circuits are both affected and only the motor and its ground are in common with both of those circuits. When the motor stops, leave everything turned on and measure the voltage on the metal housing of the motor with a test light or voltmeter. If there's any voltage there, add a new ground wire between the motor and the body. Sometimes you can get an additional clue to a bad ground if the motor starts to run when you push on it.

Also look for any wiper relays in the under-hood fuse box. Usually there's a low-speed and a high-speed relay. If the motor only quits on one of the speeds, swap that relay with another one like it. Relays don't fail real often but that's a quick way to identify one with arced or pitted contacts.