dmullen - I haven't done this but from the picture posted above it looks like their PWM had a remote dial, while the one you posted does not. That might complicate this a bit. That said the connections look simple - +/- in, +/- to motor. Again in the picture above it looks like they just took the +/- from the trailer, wired into the pwm, then sent power back out to the unit. This would retain the on/off switch, but would add in the knob to adjust speed. The extra whitish wire is for the remote knob.
Does that help?