Hey Dave... asked dad about one...
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My recollection is that when I went to the wreckers and got one:

a.  the shaft is almost impossible to remove without damaging... but has to be removed since it is where the unit fails and jams... see A

b.  the motor often isn't broken, it just won't turn that damaged shaft until it is freed up

c.  there is more that one motor/ drive that is used in these  units and they are not compatable

so... check to see if he has the unit off the car, since doing that may solve the problem if he frees up the shaft  (careful heat, gentle taps, soaking in oil)

A.  To get the unit off the window, don't break the shaft by turning the big nut.  Instead, use a dremel to cut the nut off without damaging the shaft.

If he already broke the shaft, he should take the motor apart and see if it works without the shaft.  If it does, he could try to get one off a wreck.

I had already taken the big nut off and put antiseize on it, so I was able to take it off easy.

Sitting in the garage with lots of penetrating oil on it right now.

Motor would not work and then I lost one of the little springs  .

I have the shaft free, took it out and covered it with antiseize and it turns really well, but the motor would not spin even with the shaft out.

Ixnay that message. Found the spring (1 1/2 hours on my knees in the garage). Followed the write-up from Doug, re-assembled, plug it in and turn it on......BINGO rear wiper back in service.

Thanks Doug (and Dad). I owe you a beer!