I think there are levels of DR and that all receivers have som kind of DR.
This unit (Trimble Lassen DR+GPS) require speed, direction (forward/revers) and ignition input and has internal gyro.
For BMW's there might be a difference also from Mk I-II that has this separate receiver and the later NAV computers with integrated receiver so that they have more advanced DR. This BMW receiver (65908385141) lacks that input.
Well, I'm just guessing really.
But it would be great to be able to keep some kind of position even in bigger thunnel systems with lots of exits like "Stockholm Sodra lanken" :-)
One more guess about the RXD, RXDN, TXD and TXDN. Soem Trimble devices talk NMEA as well as TSIP (Trimble Standard Interface Protocol)