its kind of a lightish blue? Wrapped in like gray tape where it's popped into the firewall? It's also runnning by itself? IF those are true then most likely then you are looking at the parking brake wire. As for testing goes, a good multimeter is your best friend when tesing wires in a car. Just to help a little anyway.
Parking Brake lt. blue - switch or BCM, lt. blu plug, pin A7
Notes: The BCM (Body Control Module) is under the driver side dash. It has 6 plugs in it, the purple plug is on the back.