What solder to use on basic automotive wiring?

60/40 mix leaded solder is best, followed by 50/50 leaded, followed by 60/40 lead free. The lead free requires a bit higher temperature, but most irons can manage that. Don't even think of using plumbing solder; use only solders for electrical connections.

Clean your connections thoroughly with alcohol after soldering to get all the residual flux out; flux absorbs moisture and corrodes anything metal around it. And minimize how far that the solder wicks up into the wires; if that happens, the stiffened, solder-filled wires will have a tendency to break off just outside the connector with long-term vibrations.