Anybody tell me which wire is positive or negative on the rear lights?

I have the 2012 model s from us. I miss when there was just a black and red wire. I don't have any tools to test the wires at the moment.
I can't remember since I have an ST now and the wire colors are different as of '14. Going from memory I think the purple ones are ground and the blue wires are the blinkers and brakes. I would still advise whenever doing any electrical work to use a volt meter to error on the safe side. Good luck.

Thanks i'll have to find a voltmeter just to be safe. I only have a multi-meter and barely know how to use it.
multi-meter should work too, you can turn it to the DC Volt (V-) which will allow you to test for power.