[shorod]

The service manual for a 1993 wagon simply states:
1. Disconnect battery ground cable, then remove wiper arm and blade.
2. Remove pivot shaft attaching nut and washers, then disconnect wiper motor electrical connector.
CAUTION: Pull connector only. Do not pull wires.
3. Remove wiper motor attaching nut, then the motor.
4. Reverse procedure to install.

I thought I had a bad rear wiper motor on my Merc. Mountaineer. After removing the nut under the plastic cover, the arm slid off the spline shaft. Once I got the arm off, I noticed many of the splines in the wiper arm were flat. I tried the motor just for kicks. It worked fine. I reinstalled the wiper arm, being sure to "torque" it down. The rear wiper's been working fine since. However, the first time I turn it on and there's some ice on the arm, I suspect I'll strip the splines out the rest of the way.

To shorten this long story, on the Mountaineer, the rear wiper arm is just slid over a spline shaft. In your case, there is probably corrosion in the splines causing it to be seized in place. Once you remove the nut, if you spray a bit a penetrating oil into the shaft, let it soak a few, then pry the arm away from the body of the car, you'll probably be able to get the arm loose.

-Rod