Don't know the "easiest" way, but one that works for me requires 5 jumper wires. Each of four of the wires has a spade terminal on one end and a female connector on the other end. Hook 'em up to the harness plug on the wiper corresponding to the same color harness location on the bulkhead. Shouldn't matter what body style, for the most part; blue to blue, green to green, red to red, brown to brown. The last wire is a ground wire from the case of the wiper motor to be tested to ground on the mule vehicle.
Works for me; the last 3-speed wiper I checked, I used my '72 B-body as the electrical mule to test an A-body wiper ('cuz obviously the harness plug would match up).
Also, it requires that your dash harness is working properly (specifically, the wiper switch).
