There is a very good tutorial about proper soldering technique for 12 volt automotive electronics at: http://www.mmxpress.com/technical/connections.htm. You want to do "butt" soldering when soldering wires together. Use heat shrink tubing over these connections, not electrical tape. Stagger the butt connections when doing two-conductor cables, and use only one length of tubing to cover both splices.
Staggered Butt Connections Eliminate Bulges |