AO Gold Member
Join Date: May 2008
Location: United Kingdom
County: Perth and Kinross
Switch the multimeter to the current range (DC amps - you might have to use different plugs on it to do current measurement), and put it in series with the battery (ie disconnect one battery terminal, connect one multimeter lead to the battery and one to the battery wire) and see what it's drawing.
Make sure it can handle 10A or so, and don't try starting the car with the meter connected.
Once you know what it's drawing, try pulling various fuses to see which circuit it taking the power.