AO Gold Member
Join Date: May 2008
Location: United Kingdom
County: Perth and Kinross
You need an ammeter (usually a multimeter with a current measurement), and use it to measure the current draw from the battery.
Disconnect one of the battery terminals, and connect the ammeter between the battery and the lead (ie in series).
Measure the current draw (don't try to start the car with the ammeter connected, btw). Then it's a case of pulling out fuses one at a time, until you find the circuit drawing the power. Once you know that, you can narrow it down to the device at fault.