Hi, I would like to know what part is in the engine bay. Location - if you are sitting in the driver seat inside the car, then front left air vent location. If you remove that front left air vent and its housing, under the housing there is a big black box with 4-5 torx screws, what is it? On the other i.e. right area, there is the location of the break fluid reservoir, but I don't know what this box does.

Also, do E60 M54 have a transfer box? Where is the location if it has any?

 

big black box = ECU

other side = brake servo. Look under it carefully, there is a rubber grommet that gets clogged up easily, which means you can end up with a puddle of water there, which can ruin the brake servo, or the water can enter the cabin.

 

Don't know about your E60 but on mine, Driver side (left) has the brake fluid reservoir and the other part you refer (rightside) is the electronics box, this box holds the DME and VVT control modules and a few relays, also has a cooling fan in the bottom.