Been there. This happened to our 1989 240 wagon. It's the wiper relay.
Sounds easy, but FINDING the relay is the hard part. As I recall, the
relay is located under a panel, under the carpet, on the floor of the
driver's side. It's hard to describe the location. There's a picture in
my Haynes manual which only made sense after I found it. You have a white
plastic handle to open the hood. If you follow that to the point where it
enters the bulkhead, under the dash, above the driver's left foot, you'll
see a mess of wires, including the bunch in the upper left corner that go
behind a black metal panel. You can actually pull that bunch of wires and
drag the relay out, but you'll never get it back in. It's easier to pull up
the carpet and remove the one or two screws that hold the black panel, then
you can easily replace the relay.
Why they put the wiper relay under the floor is anyone's guess. In my case,
I did a post-mortem on the relay and found it full of water.
