Why dont my reverse lights work when I put it in reverse?

Trikkidick West, since the reverse lights went out after changing column probable bad wire/plug connection at the park,neutral,reverse bulb switch which is attached to lower part of steering column. Strange thing is you say they are working now after plugging in trailer plug so you should also follow that harness and check for shorted/damaged wires, also make sure all grounds are clean and tight. Link below shows a diagram of switch on column, switch is #3 on diagram. Good luck.

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http://www.truckforum.org/threads/backup...