a multimeter is a tool used in electricity. it allows you to check potentials, currents and resistance between two points. in a nut shell, what you have to do is check for a 12V connection between two wires when pressing(for example) the brake. when you find the connection you would know which wires should fit to the brake light. Similarly for the other lights.

I think a good way to tackle this is use this manner to identify the power sources then pass 12v through your rear lights to identify which is which and connect accordingly ...

thanks for the pics. love the bodywork =)