That's a slick wagon, I like it!
In the past, on both my wagon and my El Camino, I've had the same issue with wipers. The El Camino was cured when I replaced the ground strap with a 12ga piece of ground wire - no problem since. The wagon would do Just as yours is doing, so I replace the ground. That worked fine until the last leg of a recent trip I took. Just like you it gave up, in the rain, at night and on the interstate. When I got back I checked everything out, but could only get it to operate at normal speed for 10-15secs at a time - no high speed and no pulse. I just replaced the whole thing, now all is well (plus a new motor is really much more quiet).
Jack is right about the vent, I had your issue myself and was very worried about the seal, because there are no more. I took mine out - very carefully - greased it up so that it would just barely move. When I got it back in I pulled it tightly closed...and keep a bat in the car for anyone who might attempt to open it!