Ordinarily I'd expect the wiring to the front and rear trucks to be connected in parallel on the PCB, with one set of connections from the pcb to the decoder. I'm sure that's how it is on my 3 truck shay, all the wiring is in parallel.
Presumably it runs ok on the track, so both motors are getting power. It's probably a bad connection/broken wire somewhere, so can you try applying power to one side of the front truck and the opposite side of the rear truck? Then swap sides? That should help isolate which side of the rear truck has the pickup problem.
Presumably it runs ok on the track, so both motors are getting power. It's probably a bad connection/broken wire somewhere, so can you try applying power to one side of the front truck and the opposite side of the rear truck? Then swap sides? That should help isolate which side of the rear truck has the pickup problem.