Have a look at the diagrams at http://makezine.com/2012/02/28/how-to-splice-wire-to-nasa-standards/ to see how to solder 2 wires together correctly.
The NASA soldering standards at http://www.hq.nasa.gov/office/codeq/doctree/87394.pdf are extremely detailed, but are worth reading if you are going to be doing a lot of soldering.
I would normally then use some heat shrink sleeving (or self-amalgamating tape on a T-joint, where you can't use sleeving) over the joint. The inside of a car (especially one with a reasonably stiff suspension like the Z4) is not a friendly place for a solder joint.
The NASA soldering standards at http://www.hq.nasa.gov/office/codeq/doctree/87394.pdf are extremely detailed, but are worth reading if you are going to be doing a lot of soldering.
I would normally then use some heat shrink sleeving (or self-amalgamating tape on a T-joint, where you can't use sleeving) over the joint. The inside of a car (especially one with a reasonably stiff suspension like the Z4) is not a friendly place for a solder joint.
absolute must have for all Z4 owners!!