Hello Padrea:

I've seen this issue in the past and experienced the same thing on my
'93 240 wagon.

In my case it turned out to be an "break" in the passenger side rear
hatch wiring harness--but one that was not visible on inspection.

My intermittent funcion was working sometimes but not at others.

I replaced the harness (about $10 from Eeuroparts.com) and all was
well after the 20 minutes required to replace the harness.

Note that I also found a problem with the ground wire on the hinge.

Also.. wires for the rear wiper are the blue and yellow, so that
should help if you want to check continuity etc.

While I'm not saying that the problem on your car is from the same
cause, since it's a '90 my bet is that the harness may not be in great
shape--and so $10 is not a bad way to see if replacement resolves the
issue.

Good Luck,

Doc