Method 1
Find the yellow and black wires make sure you don't pick the radio wire which may also be this color depending on make and model of the car. This may not work with older models. Touch them together to create a circuit.
Method 2
(Warning: Only use this technique when you need to.) Put the green and red wires together. Tap the blue and yellow wires together but DO NOT touch the bare wire at the same time or you will be electrocuted.