6-way to 7-way wiring plug

I agree. Go by wire position on the plug. And DO NOT TRUST eTrailer when looking for a diagram! There are several 7 way diagrams here. The 6 way will be different color wise.

6 way plugs are often stamped RT, LT, TM, G (ground), S (spare or brakes on some trailers) and aux. Some trailers used the middle pole for brake hot and others used S for brake hot. There are some very good diagrams online showing wire position.

www.dieselhub.com/towing/trailer-wiring.html Good clear and concisely marked.

If your trailer lights worked, your job is very easy. Remove the wires from the 6 way to test. Look for the GROUND FIRST. It will come out of the 6 way and be connected to the frame, usually by a bolt or rivet. Mark G with masking tape. note: many 6 way plugs used white for ground.

Use a 12 volt tester battery to light up the tow lighting, grounded to the frame. Disconnect the ground on the trailer battery(s). Touch each wire (after determining the ground) to the battery hot to find LT for left turn, RT, and TM (tail+markers). You can mark each wire as to what it does.

LT will be bright on the left tail light. (often yellow) RT will be bright on the right tail light. (often green) Tail and marker lights will be of lower brightness, lighting the tail and marker lights. (clearance lights)

There are only two wires left. One should operate the brake hot and the other is often 12 volt hot or open. Many types of trailers used a 6 way plug back when, so these two are sometimes switched on what they do and different colors. If you touch the tester battery hot to the brake hot, the brakes should "clunk". Also, you can crawl under the trailer and trace the brake wires to the front 6 way plug. I'm guessing the two positions are "S" and "AUX" (12V +) on your 6 way.

Here is a thread on testing trailer wiring with a battery: vintagetrailertalk.freeforums.net/thread/11040/testing-trailer-wiring

Questions welcome.