I retrofitted PDC onto my '02 X5. My instructions are linked below. I got the kit from Crevier BMW. It should pay for itself the next time it prevents an incursion into something.

PDC retrofit instructions.

If you are going to do the rear PDC kit from BMW, it is not much more to get the full retrofit kit and do the front as well. The rear-only kit doesn't look as OEM as the full kit. Skip any license plate mounted solution as it is really too high to be as accurate as the bumper mounted sensors.

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