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George --
Is this a 245 you're troubleshooting? You originally said "1998" but perhaps you mean '88.
The wiper motor is a conventional DC motor that turns a small crank through a gear reduction. The back 'n forth motion is achieved as the crank goes round 'n round, much as a piston goes up 'n down while the crankshaft turns.
In the 240s, the harness attaches to three tab connectors at the motor. One is ground, the second is 12 volts always, and the third is 12 volts only when the wiper switch is on (this controlled by the rear wiper relay).
During an ordinary sweep, the motor is powered by the "always" power. The motor has an internal cam and switch -- at the "park" position, the cam switch removes the "always" power and permits only the switched power to the motor. Thus, if the switch is on, the motor sweeps through and continues 'cause it has power. If the switch is off, the motor completes a cycle and stops (no more power).
Clear or mud?
If it's blowing the fuse during one sweep, it could be the motor is loose and wobbling -- and somehow one wire is grounding. I'd look inside the tailgate during a sweep.
I'm wondering what ills could affect the relay and cause such a problem. Water?
BTW -- the logic of operation for the rear wiper is identical to the front wipers but opposite. The front wipers are always powered and are started (or continued) by making-breaking the ground circuit, not the 12-volt circuit.
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