[trularin]

The back lights are generally a four wire setup.

If you use trailer colors, you can find the code on the internet many times over.

What ever color ground you have, make sure it is tied to ground at more than one or two places. There will be a single wire that is used for running lights and allows the lamps to be in a less than bright state. the other two are for each side, one for right turn and one for left turn.

If you are trying to trick through the electrical with less than the four, you might have a number of problems that will come up. Back feed is one, others may include excessive current draw and light failures.

May I suggest you wire the rear of the car with the four wire system. If you feel inclined to use a diode, it had better be a good size one. the 1157 bulb can draw a few amps when fully lit and it is hot.

The running lights are common to the front and rear and, without the proper power management, will feed the fronts as the rear is activated.

Hope this helps.