You should have wiring in the back, left-side of your trunk, located here.

You might need to dig around for it a bit. But make sure to find the 3 pin and 6 pin. They might be wrapped up in a felt-like wrap.

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2003 BMW X5 3.0i -- MT5, 3.64s final gears, H&R lowering springs, K-Mac bushing kit
2007 BMW X3 3.0Si -- MT6