You need a schematic or wiring diagram or someone that has an idea where the wire is from, beyond my knowledge.
All I can SUGGEST is to bare the two ends of a short piece of wire and jam it into J4 #8 and the other end into the wire of connector J5 #3; That way the wire is fused at 5 amps and if the brown wire circuit is faulty (J4 #8 - grounded) it will only blow a 5A fuse (F28). Jumpering this should pick up relay K1 when the ignition is "on".
This is ONLY a suggestive idea, something I would do (or similar) to keep going down the road. You
do need Thors/Freightliners help with a schematic.