In 1998 Rangers there are 2 relays in the engine fuse box for wipers
These Relays are controlled by the GEM(generic electronic module)
When you turn on the wipers with multi-function switch that signal goes to the GEM
The GEM then activates the Run/Park Relay to send power to the High/Low Relay which sends power to the Wiper Motor.
You can test the relays with ohm meter and 9v battery
But there is no over all test that I know of for the GEM, if you have/get a Ford Service manual it will have circuit tests for GEM connectors, but there is no plug and play test for them