I had the same issue on my '63 (12V) car. Fairly heavy current pull, very little movement. I took it out and the problems I found were:
1) 40 year old dried up grease. Cleaned everything and lightly regreased
2) The brushes on the motor were still good, but conductive gunk had built up between the contacts, so current could shunt around to the other brush that way, instead of going through the windings. Cleaned out the gunk and very lightly polished the commutator again

Works just fine now. The parking works too, once you have it apart the switch on the gear box is fairly self-explanatory - it might just be sprung open, or the contacts may have built up unconductive gunk.

I once grabbed a 2 speed 122 wiper mechanism with the thought of swapping this into my 544 somehow, but there was nothing common between the two, it didn't seem to be reasonably feasible, so I left the PV's 1 speed alone.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)