I just changed the wiper motor on a '95 Sedan Deville today.
Three bolts hold it to the firewall. The bottom one is tricky, because it is hard to see what you're doing, and it'* easy to drop the bolt into dark crevass' if you're not carefull.
Getting the arm popped off the knuckle is tricky. It will be easier if you have a helper.
You almost have to use two pry bars, and use one to push off of.
The motor on this Caddy was only working on HIGH speed, and then it would only move the wipers about three inches, up & down. Nothing would happen on LOW speed or DELAY. I don't know if a different motor malfunction would cause the parking problem and otherwise work, but I wouldn't be at all surprised. I was looking at a '95 LeSabre in a dealer lot last week, that had the exact symptom as goldfinger described.
And if you get a motor from a wrecking yard, plug it into your harness and make sure it works & parks correctly before you bolt it in. We got a replacement from a yard, and it did the exact same thing as the original was doing on the Caddy. And it was from a 1998 car. Had to go back to the yard and exchange it for another. The one that worked was from a 1990 car. So you always have to check to make sure they work OK.