I'm not quite sure what you are asking. "Does the backup light wire (or brake light wire) wire split off from the marker lights?" That question makes no sense to me, so I'm misunderstanding what you are thinking. Maybe the following will help - or maybe not. Hope so.
From the TM's Bargman 7-pin connector, there are 7 wires in the big round black cord that goes to the area in back of the refrigerator. These 7 wires perform the following 7 functions, and they are independent of each other. No sneaky cross connections. (By the way, the numbers in this list do not correspond to the positions in the Bargman connector.)
1. The wire to the running lights comes into the TM behind the refrig on a single wire.
2. The wire to the left turn and left brake light comes into the TM on a single wire.
3. The wire to the right turn and right brake light comes into the TM on a different single wire.
4. The wire to the backup light comes into the TM on a single wire.
5, 6, 7: The other three incoming wires carry electric brakes, 12-volt supply, and ground.
You seem to be having trouble with running lights (number 1 above). If wires #1 and #7 are properly connected within the TM, none of the others should be involved. Don't allow them to confuse things. Sorry if I have misunderstood the question.
I don't think you have yet told us if +12VDC is present at any of the non-functioning running light fixtures. With the running lights switched on in the truck, you can start by checking for +12VDC on the incoming running light wire (#1 above) behind the refrig. If voltage is not present there, then you have a problem with the Bargman connector, or the big black cord from the Bargman to its end behind the refrig. If you do have voltage there, open up one of the running light fixtures, pop out the bulb, put one voltmeter probe on the center contact of the socket, and the other probe on a KNOWN GOOD GROUND (not the ground in the non-functioning socket). If you find voltage there, then you have a bad ground in the running light circuit. To confirm it, attach a clip lead or other temporary connection from the shell of the running light bulb to a KNOWN GOOD GROUND (not the ground in the non-functioning socket). Do any or all of the running lights come on?
Bill
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