What's the module in the rear cargo door? By the spoiler?

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'02 X5 4.4 - 67k miles
'01 RX300 - Wifey's, 74k miles
'92 325i - My classic (Dinan S1 susp, clears, M3 kit, M3 exhaust, M3 Double-spokes, 216k miles)
'93 318is - Gave to niece