Not sure about here but in the UK they have plastic connectors that will allow you to connect into an existing wire. The connector kinda pushes the insulation off of the existing wire and connects the new wire at the same time.

Dragged one out of my old Toolbox I brought over from the UK. This one is made by 3M and the packaging describes it as a Scotchlock connector. Should be just the job if you can find them here in the US.

Sorry the photo is so blury, camera = piece of crap. Roll on my new digital rebel for Christmas.