Greetings and Happy Old Year!

I recently added a cargo trailer to the fleet and towed it home with 'Ole Blue'
At first it took a little coaxing to get the running light operational but eventually I think the rust wore off all of the ball and receiver and made a functional ground connection (though I had a solid [ HAVE ] ground through the trailer wiring... so that probably wasn't it. Anyway fidgeting and they eventually worked.

Half way back, about 100 miles and something happened to the running lights and now my truck's round plug has low voltage on that pin of the 7 prong connector.
Thanks to this forum I have the FSM and within I found schematic 8W-54 that covers the circuit.

BTW, the brake lights (and turn signals are lighting full intensity)

BUT,
Historically, does anybody have "THE MOST COMMON'' weak link in the system and perhaps a troubleshooting technique other than check the fuses, swap some relays with something you know works, etc

How about the FCM (front control module) how could I test to know if it is working as designed?
The IPM, is the IPM (or TIPM [not sure where my 1500 SLT has 9total or only integrated] ) fuse box assy. Is the IPM a common fault with these trucks?

Cheers and thank you in advance.

 

The usual suspect is the junction where the trailer harness hits the rear harness. Dodge really skimped on that.....

 

wow, that was quick, thank you

so right at the rear of the round 7 pin bumper connector? or is this said junction located further forward?

Hey You, thanks...

 

Further forward, where the trailer harness breaks out from the rear harness.... I *think* its forward of the axle, but, it's dodge, so, could be anywhere.

 

Update: now working.
I found on 'the new to me' trailer... that someone had replaced some of the marker lights but was using the wrong wiring scheme. Where the old lights had two wires (isolated) the new lights had the one terminal 'also' grounded to the framework and wouldn't you know it, they had the 12 vdc onto that earth grounded terminal. on a few of the lights.

So, dead short on the parking/running trailer lights...

Then I found a melted (but not blown) 30 amp fuse stuffed in a 20amp slot in the IPM for 'trailer parking lights'. Again, melted but not blown...
BUT, I had no power for the trailer parking lights on either the round 7 pin or the flat four pin underneath ( Brakes/Turn signals worked fine)
THIS is what I was asking about, here...

So trying to chase the fault, find the next connection forward that HeyYou spoke of, (I think it would be under (actually atop) the fuel tank) and I tested and tested and all of a sudden it started working.

QUESTION: could the FCM or something in the IPM work as a self resetting breaker?
I replaced the melty fuse with the correct 20a (yes, checked the melty 30 and it still had continuity)

Oh, and the trailer lighting circuit had an isolated neutral ( - ) neg and only when those other than original lights were installed, was any trailer lighting wire connected to trailer chassis.
At first connection months ago, all lighting was working. then about 100 miles down the road the running/marker lights started getting dim but the brakes and turns worked full bright. So as I hauled, the trailer ball and receiver got polished and the electrical 'short' connection became better (worse actually) between the trailer short circuit and the truck chassis...

* in trying to troubleshoot/test the truck towing electrical, the trailer (any trailer) was not connected.
... and suddenly it all started working.