If you put in connectors it will be a place for possible intermittency. Loss of signal. Corrosion. You don't need to worry so much about service being required for wires. A straight run is best. Don't know what you think needs serviced but you are wasting time and money. If the components/lights at the ends of the wires fail, you replace the component/lamp/etc. If you have to replace a whole wire, wire is cheap. There are no maintenance issues that require extra connectors on an old non-electronic truck.
If your wires are too short, you should notify the harness supplier and get the correct harness. This is a Red Flag to me that you are using the wrong wires, harnesses are extra long, like 12 feet long from the fuse box..
Otherwise, if you add length, it should be done by intertwining/twisting the strands together, soldering and cover with shrink sleeving and liquid tape over that.
I would like to know where you got a harness that needs extended so I don't buy one from that company. The harness I have used are designed for long bed trucks. Jeggs and others say "extra long" on their harnesses. Speedway says "Taillight leads measure 17' long".
Taillight wires are generally; one wire for running lights to driver rear. Then chain to other side. Same for stop. Two seperate wires going back for left and right turn sigs.
When chaining, or tapping into a wire, or when connecting two ends together, you can use soldering/sleeving, a solder splice or any number of crimp connectors. These solder or crimp splices should also have heat shrink sleeve over them. Any of these methods are used by us all and are plenty adequate for auto wiring. It ain't no rocket. Wire routing, up and under, away from splash and well clamped, wire tied, loomed ....are good ways to complete a good looking and long lasting job. Whatever you decide is up to you. I recommend you study up on the method you choose. The internet wiring sites/how-to's are 50% good info and 50% wrong. Just like any other issue you look up, read a whole bunch to get the real feel for what is accurate.
