windshield wipers don't work

[Bluerb]

Howdy. I'm new here and will post an introduction with photos of my new burb this weekend. For now though, I've got a question about the wipers. I'm buying a friend's 1990 GMC 1500 4x4 suburban, getting it this weekend. It's been parked in dry central Oregon for a couple of years and is in mostly great condition.

She brought it back here to rainy Portland last week, and discovered that the wipers won't run except with the washers pumping water. Since they work fine when the washers are going, it doesn't seem that the motor is bad. I'm thinking it is likely the switch, but I wanted to see if anyone here has other ideas.

Are there specific wires that could come loose and cause it to only work on one setting? I haven't been under the dash yet to look at the setup.

Thanks, any thoughts are appreciated.

Timothy