What, and where, is the Pressure Control Solenoid "C" on a '98 7 Series.

This is my brother-in-law's car. He's reporting that the car does not shift
properly -- I've not ridden in the car and I cannot describe the physical
symptoms -- and he says it shakes. The Check light came on, and he pulled
codes. He reset the codes, drove some more and the symptoms persisted and
the Check ultimately came back on. Pulling codes again gave this code as a
repeat -- it appeared on both lists of codes.

Is this electrical part one that lives deep inside the transmission, or does
it reside on the outside where we can service it ourselves?

I'm searching Google as you read this, but so far I've only found reference
to the OBD II code list, I'm looking for a repair strategy.

Thanks.
Jeff