[X1Denver]

Just found this note.... I looked under my car and there it is. Its a combo aerial and RDC module.

Tyre Pressure Monitor (RDC)
The Tyre Pressure Monitor (RDC) is a system which monitors the tyre pressure while the vehicle is in motion. It is offered exclusively in the USA (legal requirement) and as optional equipment (SA2VB).
The system of the new generation (Generation 3) now consists only of 5 components: the RDC control unit (control unit with integrated reception aerial) and 4 sets of wheel electronics.


RDC control unit
Assignment of wheels
Axle assignment and identification of direction of rotation can help to assign the Identification Feature (ID) of the wheel electronics to a defined installation position.
Axle assignment:
The RDC control unit is installed close to the rear axle. For this, the messages on the wheel electronics from front axle and rear axle differ to varying extents. The averaged signal strengths of the wheel electronics systems thus indicate the axle assignment. Since the RDC control unit is installed to the rear of the vehicle, the wheel electronics on the rear axle receive signals at a higher level of receiving power than those on the front axle. Axle assignment runs parallel in time to the idenfication of direction of rotation. In the calculation of signal strengths, only wheels rotating in the same direction (on each side of the vehicle) are taken into account.