If the proportioning valve circuit in the abs unit is clogged or messing up it will also cause the abs to go nuts when braking. Most cars the proportioning valve isn't inside the abs unit but a separate part and when it messes up it will cause the abs to activate erratically since it causes front or rear to lock up since the brake bias is off.

Just something else to throw at the mix.

Best way to catch this problem if that is the problem is to disable the abs and see if the car locks up a wheel to easy.

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Aftermarket E53 Radio Install