trace it back, it should be going to the transmission, its what modulates how hard the transmission should shift.

a white line also is near the brake booster servo assembly which goes to the rear headrests. It branches off at a small T shaped manifold right behind the brake booster servo and goes into the cabin.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)