Trailer harness miswired

[Mario]

Got my M998 few months ago. More recently I bough M1101 trailer from a buddy.
Connected it to my rig and couldn't get lights on trailer to work right.
Buddy swore that he tested my trailer on his M998 and it all worked fine.
Took several weeks for him to get to my house (his rid was rented to filming studio for upcoming movie).

Fast forward to today: hooked up the M1101 trailer to his rig and all the lights, blinkers, etc. work flawlessly.

With my M998 I get weird output, like no running lights, blinking is on perimeter lights, etc.
Looking at plug harness, it looks all original -- no splices, knicks, etc.

You guys run into this thing before?

Where is the other end of the harness to check if wires are messed/modified on that side?

I can obviously splice wires and rewire it to work right, but trying to understand how this happened in the first place, before I start screwing it electrical.

 

[Action]

I would try a different light switch first. Are you using a push button switch?

 

[emmado22]

Check to see if the vehicle end socket is rotated......

 

[Mario]

I would try a different light switch first. Are you using a push button switch?

Original, manual switch.

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[Mario]

Check to see if the vehicle end socket is rotated......

I will check, but I'm pretty sure the plug is keyed to only go in one way.

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[juanprado]

Check to see if the vehicle end socket is rotated......

I 2nd that! My 5 ton was driving me crazy with trailer wiring problem. After a lot of grief and time, plug was rotated 180 out. From years of forcing it, the alignment notch was worthless. Letters were also not in alignment and that us what caught my eye.

Get test light and pin out diagram to verify, don't assume anything.
By the paint on bolts, I think it left am general like that.