There were two types of plastic slip cover on the arm connection to the motor unit. An early type with an inspection hole and flap and the superceding type that does not have the hole and flap.
The 'clamshell' metal head on the gear unit can only be removed with the whole gearhead mechanism out of the car. The 'clamshell' is plastic and supposedly removable in situ on the second facelift cars - those with an inset grille.
Stiff and slow wiping will be down to dried up grease - it turns to hard wax - inside the gearhead. I had to strip and re-grease a 1992 wiper unit about 5 years ago to cure that. A short term fix is a few drops of 2 in 1 oil on the metal rod that the wiper arm bolts to.
I stopped the wiper when it was vertical on the screen, lifted off the plastic slip cover, slipped a rag underneath to protect the glass and oiled the shaft lightly.

The car I worked on had a Bosch motor and gear unit, although the spare motor we bought turned out be an SWF item.