I'm going off some old memories, but I had my 3 speed wiper on my Cuda break on me back in the '90's so that it would not go into Park. It was fairly easy to pull apart and figure out what the problem was, and I found the part I needed to repair it in a junkyard. Here's what I remember:
There is a nylon gear inside the wiper unit that has a track in it, and when the wiper is in one of the 3 speed settings, the gear rotates in one direction (at different speeds for each setting), and the track in the gear connects to the arm on the outside of the wiper unit, which connects to the wiper linkage and causes the wipers to go back and forth. When you turn the wipers off, you are not killing the power to the wiper motor, what is happening is you are reversing the motor, which causes the nylon gear to reverse. The wipers will instantly stop and move in the other direction when this happens (if the wipers were halfway through their sweep going down when you turn the switch to the off position, they will instantly start moving up). There is a small tab in the track in the nylon gear, and when the internal piece that connects the gear to the outer arm hits this tab, it causes it to go into a small 'side' track that has a shorter radius than the 'normal' track. The gear is orientated so that this 'side' track comes in play when the wipers are at the bottom of their travel. Going into the 'side' track is what actually parks the wipers - the shorter radius of the 'side' track causes the wipers to go down further in to the park position. Once it goes a short distance down the 'side' track, it will bump against a set of breaker points, opening the points and killing the power to the wiper motor. The 3 'forward' speeds for the wiper motor do not go through the breaker points, so when you turn the wipers back on, the wiper motor spins in it's 'forward' direction, it will come out of the 'side' track into the 'normal' track, and the wipers will run back and forth.
What happened when mine broke, was the small tab on the nylon gear broke off. When I turned the power off to the wipers, the motor reversed, but it never went into the 'side' track, so the wipers just went back and forth in reverse, never parking and never shutting off. And, wipers going back and forth in reverse look just like wipers going back and forth in forward, so it was very hard to tell what was really going on until I took it apart and reverse engineered it. When I figured out that the tab had broken off of the nylon gear, I found another gear in a junkyard and replaced it, and my wipers have worked fine since.
Hopefully this makes sense - it's a long winded explaination that would be much easier to explain if I had a picture of the nylon gear so I could draw arrows and circles on it
i think the variable/3speed parks with the stuff thats under the lid/a kind of breaker point attatched under the lid and a jadajadajada.its all in the service manual,even different tests you can do to check whats wrong.also i think PASSIONFORMOPARS.COM has some info on her site how to test the motor.if you not have the oem wiring i would check that out.also do you have all screws in your left interior pod that holds the switches,it must be grounded. 
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