You said there are only 3 wires. Is that 3 wires in and 3 wires out? If it is only 3 in then they are certainly not positive, negative, and ground as the switch would just be used to complete the circuit.
If you don't own a multimeter then I suggest asking a friend over for dinner and have...
The model number is listed on the device as "WH7016C". I suggest trying to find a manual for that device which would probably have a sample hookup diagram. If you are guessing on how to hook it up you might damage the device.
Dale
http://www.activeenterprisesltd.com
You can probably buy a inverter for close to what I might take you to build it from scratch - that is unless your labor is free and you really enjoy building it yourself.
Dale
http://www.activeenterprisesltd.com