In the remote PDM behind the driver seat, Take out the OPT/DIAG breaker or fuse out see if it kills the power to those circuits. I want to try that first.
Since it is trailer marker related, I am also wondering if something in the trailer wiring is causing this. I will supply you with some wire drawings I have of a 2004 columbia. Also, a FUSE PANEL schematic so you can use it to figure out the wiring.
Freightliner drawings are somewhat hard to read. You have to look at the break down and the circuit descriptions, Look at the From and TO chart. That will tell you circuit numbers, and pin locations.
1st find your circuit, then go to the From and the To. It helps if you print these out and spread them on a table. Get you a pen and piece of paper and write down pin locations.
When you look at the PDM schematic. Pull the instrument fuse again, check power on one those legs, with the ignition off, if you are getting power on one side of the leg, then it is back feeding from somewhere. Click the blue links for the Dash wiring, and the PDM schematic. It will take you to MediaFire, then it will take you to the Download now button, and you will have pop ups. I had to upload it here because the dash wiring was to big to upload it to JustAnswer. I checked the links, and they do work.
Dash wiring and PDM wiring.
Edited by Clayton on 1/21/2011 at 5:31 AM EST