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Trailer hitch wiring doesn't sound like it should be complicated, but it can be quite a mental challenge as you will see.
I recently completed a DIY trailer hitch for my car and needed to wire the lights to go with it. The problem is that my car is one of those that have separate bulbs for the brakes and the blinkers. My trailer has only four wires going to the lights, ground, running lights, and a left-and-right light, which act as both brake and blinker lights.
At the time of wiring these lights, I had the taillights of my car apart and didn't want to run to the store to find a solution. Instead, I simply wired four diodes, which would isolate the outputs of the brake and blinker and allow me to combine them. This circuit worked, and low and behold, the blinkers blinked and both brake lights came on when prompted. 
Shortly after, that I realized that the simple circuit had a flaw. The issue was that when the brakes were on while the blinker was on, the blinker wouldn't blink. (Despite the flaw, my simple circuit did work well enough for my trailer to pass inspection since they never had me hit the brakes and turn the blinker on at the same time.)
The normal function of the lights should be that when you hit the brakes, both lights come on; if you turn on the blinker with the brakes on, that side should blink, while the other side stays steady on. The question became how to accomplish this with as simple a circuit as possible.
Instead of just posting the solution, I would like to make this into a little challenge: design as simple or as elegant a circuit as you can for the application above that would make the lights work correctly. By simple or elegant, I mean least number of parts and/or lowest cost to produce.
I have a number of what I feel are good, simple solutions that would work and I think there are likely many good solutions I don't have. I also have some designs that seem more complicated, but would work. So please, post your solutions. Next week I will discuss and post the solutions I have – if they haven't already been guessed.
By the way, I ended up buying the circuit at the store, which cost me about $15. The low cost doesn't really make it worthwhile to make this (should cost about $2 in parts), but its fun to know how it works.
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