The intermittent rear wiper uses different terminals for wash and run, because the intermittent function is built into the motor itself. If it washes, at least the motor is working, but you need to determine whether power is getting to the run terminal on the motor in order to find whether it's the motor or the switch.
Later wipers have a motor that's put together with rivets, making them hard to fix, though not impossible if you can file, drill or mill the rivet heads off. If you're careful, you can do this and put it back together with screws.
If your wiper is getting sluggish and slow, it may be worthwhile trying to open it up and pull and lubricate the wiper shaft before it seizes up and burns out. The shaft is long and corrosion works its way down. Once the motor is apart you can just push it out, clean off the rust, and grease it. Older ones with screws are well worth the effort if they have not stopped altogether. Once the thing gets too stiff it will either burn out the motor or strip the nylon gears.