Another question before I start. I have to solder up to three wires to one wire - two headunits and source switcher to the wiring harness. Is there a proper way to do this? Twist three wires around the one and solder? Strip the harness wire way back and attach the other three at different spots? Then cover the entire mass with heat shrink, but won't it split because it can't seal the end with three wires?

I've finally got (most) all of the equipment, but hardly any knowledge.