2000 Ford Excursion wipers on-off

With no test results to analyze, the best suspect is the GEM Module. I don't know where that is on your vehicle, but on Ford pickup trucks it is real common to have water drip, from a leak at the windshield, onto that module and cause all kinds of weird problems. You might look for that module, then see if there's signs it was wet. Also unplug the connector and check both parts for signs of corrosion between multiple terminals.