how to find wires?

This might be a silly question, but i don't know how to find the wires eg. ( - ) door trigger wire. How can you tell using the multimetre? do you connect the black lead of the multimeter to ground and the red lead to the wire you think is the ( - ) door trigger wire and if the multimetre reads  - 12 volts when you open the door , then it's the correct wire?

srry close but no spliff set meter to DC voltage (12V or 20V) connect (+) lead to 12V contast prode wires with (-) lead until meter shows 12 volts when tiggerd in somes cases (ford) its a positive tigger so connect the (-) lead to ground and probe target wires with (+) lead untill meter shows 12 volts make sure to the wire u find cathces all the doors

Thanks

You might want to check out this thread. I asked how to use a DMM a while back, and this thread has a couple of good sites on it.