The switch circuit for the light comes out of G104, then to C132, then to the backup lamp switch, then back to C132, then to the TIPM thru C7. The TIPM is looking for a ground to turn on the lamp.
The left lamp power comes from the TIPM thru C8 as a wt/lg wire, goes to S120, stays a wt/lg to C135, then to S325, then to the lamp connector. The ground goes from the lamp to C135 as a bk/dg, from there as a bk to G104.
The right lamp power comes from S325 as a wt/lg, then to lamp connector. The ground comes from the lamp to S309 as a bk, then from S309 as a bk/or to S308, then to C135 as a bk, then to G106.
I hope this helps.
C = Connector, S = Splice, G = Ground.