86 GMC Sierra Classic 2500 questions

[gmcnoob]

Well, I checked all of the grounds. Loosened them and hit them with a wire brush real good and reattached them nice and tight. Everything works including the license plate bulbs which is a first. Now it doesn't matter which turn signal I turn on all of the rear lights flash including the license plate bulbs. Everything in the front is working correctly. Brake lights work the way they're supposed to, reverse lights work. Either turn signal has all of the rear lights flashing. I am baffled.

The only electrical things that I have ever messed with on this truck were replacing all of the fuses and the headlight switch.