Finally solved the mystery of what goes into the empty engine bay compartment

[Galapolis]

Apparently it's for the electric water heater, which was most commonly found on diesel models but could also be optioned from factory on a 330i.

Part number: BMW 64.12-6 904 668

Picture (this one depicts a 330i):

37307

[BMWCurves]

Good info!

Also for the Xi models it housed some sort of hydraulic system related to the AWD system.

[Oli77]

Did not know that. Thanks.

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[holyc0w]

318i has the battery in there.

[t.er]

I also thought some models had the ABS module there

[sillieidiot]

Yeah i think some of the older pre-facelift models had their ABS in there. I never thought it was a mystery lol

[Karl Lazlo]

Early M3s also housed the ABS/DSC in the drug bin, as well.

[Will]

The RHD models had the brake booster, master cylinder, and ABS stuff in that passenger side bin.