Check the bulb on that corner. You wouldn't believe the stuff I've found done by previous owners even with things as simple as light bulbs.
I've found 1156 bulbs in 1157 sockets... You really have to force it to make the 1156 bulb fit in the staggered 1157 socket. There's someone else on the board that just found one of those so I was reminded of my incredulity at finding that particular bodge...
I've also found a tiny piece of copper wire between the lead bumps on the 1157 tail lamp bulb in my parents' G20 van. It was held in place by grease. I believe this one was from a dishonest mechanic. I found it after a failed NYS safety inspection. I can't say for sure the guy that did the inspection did that but I never took him any more work either. I found out several years after that he had a reputation for failing cars on the first go so he could double dip.
My 2005 Silverado 2500 HD came to me with four very burnt out marker bulbs. It passed California Safety inspection the week before I bought it. It passed NH safety inspection with them still burnt out. They're working now. So much for the nanny LOL
The LH rear turn is a single wire. If you download the GM wiring diagrams it'll have the color.
In the rare case that the plug is damaged or very commonly someone bodged the wire with a scotchlok or butt splice or ... and the damage is isolated to the plug area you can use the 1985 and later wiring pinout and replace the molded connector with the dead reliable 2X2 4 position Weatherpak that GM used on the 1985 and later trucks.
The taillamp disconnect to the firewall plug is one of the simplest harnesses on these trucks. Four 14ga SXL wires with Weatherpak terminals at the rear and Packard 56 terminals at the firewall. Replacing this run gives me an excuse to run a 10ga RED SXL trailer breakaway battery wire to a breaker next to the J-stud on the firewall and 12ga Blue SXL trailer brake wire to the cab from about 12" past the rear bumper at the same time as the tails. I usually cover all six wires in split poly loom.
If you need the actual taillamp harness from the LH frame rail to the bulb sockets I believe this one will do the job with minimal work...
http://www.mrtaillight.com/product_info.php?products_id=673
I would cut off the bullet plugs for the license bulbs and install single position Weatherpaks instead.
I would also extend the ground wires to the frame using Solder and heatshrink or single position Weatherpaks.
I avoid the GP PVC jacketed wire sold in the auto parts stores. It's not very abrasion resistant, gets brittle when it's cold, and it cracks with one or two years of weather exposure if nothing else kills it.
You can get Crosslink Polyethylene automotive wire in 8 or more assorted colors in 12, 14, 16, & 18 gauge. Ebay outfits like the Wirebarn and some other online vendors sell 8 or 11 or more assorted color 25' coils, what I call project paks, of Crosslink Poly wire.
The larger commercial wholesale outfits like Del City and Waytek and ... only sell 250' and maybe some 100' spools. Way more than you need for a small project.
Crosslink Polyethylene primary automotive wire is denoted by SXL, TXL, & GXL. If you look at the GM wiring diagrams you'll find that GM used mostly SXL wire.
The Weatherpak parts are around $15 from Mouser Electronics The Packard 56 Male terminals are usually around $0.25 or less each from a lot of different vendors
.
I usually pad the terminal and seal count out by a few JIC some roll under the bench or I screw up crimping one or two.
Gender on electrical connectors can get strange...
Female Weatherpaks are female terminals inside a Male tower plastic shell.
Male Weatherpaks are Male terminals inside a shroud plastic connector shell.
Qty..Mfr. #.....Description
4....12015791...Automotive Connectors 1Pos FEMALE BLK CON ASSY 20 AMPS
4....12010996...Automotive Connectors 1Pos MALE BLACK 20 AMP WEATHER PACK SHROUD
1....12015024...Automotive Connectors 4Pos MALE BLK 20 AMP WEATHER PACK SHROUD
1....12015798...Automotive Connectors 4Pos FEMALE BLK CON ASSY 20 AMPS
20...15324982...Automotive Connectors INDV. LOOSE CBL SEAL GREEN
10...12089040-L.Automotive Connectors MALE TIN PLATED CBL RANG .80-.50MM2
10...12089188-L.Automotive Connectors FEMALE TIN PLATED CBL RANG .80-.50MM2
