It's hard to tell from the picture what is going on with all these wires, but this is a shot taken in the middle of connecting the 1992 ctd wiring for the back of the truck to the 1977 wiring that is already in the back of the truck. I used a Haynes manual wiring diagram to figure out the color codes of the chasis wires. Since my cheapo manual does not cover diesels and I have not coughed up the 100 clams for the Mopar Field Service Manual for the 1992 cummins trucks, I had to use deductive reasoning to figure out what is what on the 1992 side. As I traced each wire, I labeled it with tape and marker. When I had them all figured out, I soldered and shrink-tubed all the connections. The frame harness on the cummin/dodge side originates at a dark blue plug under the dash to the left of the steering column. It then routes through the firewall in a 1 1/2" grommet, goes down the firewall, and back inside the left frame rail. When I dismantled the donor truck, I cut the harness at the bottom of the firewall, so the first thing I did was retrieve the down-stream part of the harness out of the old frome. Then I worked my way down the loom and identified the purpose of the wires as the exited the loom. The first wires I came to were for the fuel tank so I labeled those wires for the ground, guage, and low-fuel light. The next was for the abs sensor on the differential. These are not needed so I bundled them together and labeled them "abs." Then came the abs dump-valve. They were bundles and labeled "abs." Then I got to the left lights. The 92 ran ground wires instead of grounding to the frame. The black wire was ground for all the grounded items, that left three wires. One had to be the stop/turn, another would be tail, and the last would be back-up. I bundled these in masking tape and labeled "left." moving down the harness, I came to the license plate light wire. It was the same color as one one of the three wires in my "left" bundle, so I pulled that wire out and labeled it "tail." Finally, I got to the end of the harness. Here I had three wires plus a ground again. I knew which was the tail color. That left two. One was the same color as one of the wires in the pair that was left in my "left" bundle. This had to be the back-up light wire. The remaining wire in the "left" bundle had to be the left turn/brake wire and the remaining wire at the end of the harness had to be the right turn/brake wire. These old American trucks used a single 1157 two-filament bulb to function as tail, stop, and turn lights. Dim for tail and bright for stop/turn.

Before I soldered everything up, I double checked using a volt-meter. Headlights on = power to the taillight wire. Left turn switch on with key on = intermittant power to the left turn wire. And so on...

The amazing thing is in the end, it all worked.

Note: Since there were not enough wires running back, I chose not to wire the low fuel light.