[JCC]

Sounds like a sticking relay that the switch activates. I don't know where the relay is on a '90 but will check my '92-'93 diagram.

 

[JCC]

According to the manual it could be a loose connecter. these wipers are controled by an electronic control modual called the wiper control modual(wcm)so a lose wire means it won't know whats going on. Second is a bad motor or WCM. Third a bad switch. The WCM is located above the throttle pedal, looks like way up there. Looks like the easiest thing to do is ckeck all the connecters for a start. Ihave the trouble shooting procedure and diagram if you need it.