undo the earth cable from the battery

put the multimeter between the end of the earth cable and the battery negative, with the meter switched to measure current

remove fuses from fuse box one by one until current useage reduces to almost zero

find what that fuse controls, and, after replacing fuse, unbolt/undo/unplug/remove as appropriate items on that circuit until current useage drops

if none of the fuses in the fusebox affects the current useage, there are more fuses behind the glove box