Hello, on my 03 E55 the audio gateway fuse is number 6 located in the left side (as you look from rear) of the boot, behind a trim panel, when that small square panel is removed, you’ll see a black rectangular plastic cover, remove that and you will see the fuses, number 6 is a 40amp fuse. You may want to remove fuse 9 which is the overhead control panel 25amp, another known drain. It could also be the seat adjustment fuses 23&34 both 30amp located drivers side (uk) dash side panel, accessible when door open. That should give you some idea, although yours may be in a different location.