It has been suggested, but, pull all the fuses. Put your meter on milliamps as instructed above, remove the ground cable, put one probe on the negative terminal and the other probe on the end of the cable you just removed. The meter should read zero if the problem is under the dash....does it?
If it does, put one fuse in at a time, until the meter shows a current reading.
Then you should know what circuitry is involved and we can look at the book and make some suggestions