EDIT: please disregard any info provided on the grommet. 49-50 vehicles are apparently different than earlier versions.
Getting the arm and motor in sight really makes it all come together as to how simple it is.
I see Young Ed mentioned his not working well now, and possibly tight grommets. If yours looks like the pic I posted you just need to be sure that the grommet does not rub on the part of the motor that pivots, and possibly on the clip side. If it has the brass nut/swivel/insert, as long as the grommet goes around it you are good. Nut/swivel/insert does all the work.
EDIT: As far as the arm goes, I believe the wiper post/pivot that goes out through the cowl has to come out as well. That is the way it is on my 48, and also on the Chrysler that I got my donor electric motor from.