Not sure about the 1990.  It depends on the plug.  I am not sure when they changed.  I grabbed a setup from a mid 80's F150 years ago.  The switch did not work, the F150 shaft was too short.  I believe by 3/8" or so.  I tried to put the non intermittent Pinto switch shaft on the F150 intermittent switch and I never got it to work right.  I ended up getting the correct Pinto intermittent switch.
 
The box however seemed to work, just the wires were about 12" shorter than the Pinto box.  I put foam around it and zip tied it up so it would not rattle.  The original Pinto box mounts to the fuse panel.  The only thing is that the wiper pump never worked.  It needed to be repined.  I never did it as shortly after the box went bad and started doing wierd things.  By that time, I had an original Pinto box that I put in and everything worked ok. 
 
So if you have all the right parts it is pretty much plug and play.  Replace the switch, and connect the switch ground.  Then just plug the box between the wiper motor and your switch.  Mount the box and you are done.
 
I have one in all my cars.  It is an option well worth installing if you get a lot of rain.