Mine wasn't working either when I bought it so, after checking the fuse and relay and then using a circuit tester made sure there was power getting to the motor itself (see below about tailgate being shut), I removed the motor. I stripped it down and found out it was well and truly seized with rust and muck - would not budge at all.
Not wanting to fork out £80 for a new one, or risk a second-hand one for half that, I cleaned up all the working parts (it dismantles fairly easily), paying special attention to the brushes which were looking a little tired, then re-built it and hey presto! Note of caution - if you do decide to take it apart, be careful when you remove the 2 screws that hold the casing on the dynamo, as there are a couple of very small plastic clips that retain the springs for the brushes and these have a tendency to fly off and disappear into the depths of your garage - I speak from experience and had to use the spring from a retractable ball-point pen as a replacement. They worked a treat!
As the other guys have said, you must have ALL doors shut when trying it out, especially the rear tailgate as there is a safety cut-out on the interior light switch which ensures the wiper doesn't function when the door is opened. It's apparently to prevent any damage to the wiper, or at least that's what I have read
It was, i have to say, a most rewarding experience getting it all up and running again. All for the cost of a bit of WD40 and some emery cloth
Good luck!