I solder and use heat shrink tubing. I'm actually going to be doing one in the next couple days for a NA to SC swap I'm working on. Also just swapped my car from SC to NA and redid the map.
I take about 1/2" of coating off the ends of the wires, twist them together so that it's a straight line of wire with no bumps in it. Solder it and slide the heat shrink over.. shrink it and into the loom it goes. If you pay attention to how GM did the tape on the harness, you can duplicate it. They went around the wire at the end of the loom once, the pulled the loom into place and the tape goes through the slit..then around it to seal it up. That way the loom never slides, because it's taped to the wire and loom both.
