Flash is an update for the eeprom ECU that you have. It could have different reasons that should actually be outlined on the Recall notice. It could have emmisions, driveability, restraint control ect changes. The body module usually covers stuff like a/c control, lighting, some restraint functions like seatbelt, power windows, power locks, chime control, depending on model.
Many modules are networked so instead of having to have seperate inputs for each module they get the information from each other. Basically the calibration in the body module changes how it deals w/ the different inputs and how it provides it to other modules.
For an example, I had a Ford Ranger the other day that was shifting poorly (transmission). The fault was the dome lamp bulb burned out and shorted itself. It blew the fuse that gave the GEM module (same as a Body module, just Ford) its battery power. The reason the trans shifted poorly is the VSS signal was piped through the GEM, then it shared the signal w/ the PCM/TCM (powertrain control module/ transmission control module). Once the fuse blew the PCM stopped getting a VSS signal= didn't know when to signal a shift. New fuse+ new dome lamp bulb= good trans shifting.
Welcome to the wonderful world of Multiplexed Networking in the automotive world... <img src="/forums/images/graemlins/notooth.gif" alt="" />
IE... Go to the dealer and get the BCM (Body control module) flash programmed w/ the new calibration. <img src="/forums/images/graemlins/smile.gif" alt="" />
Really the letter should state why the calibration is needed. CRDs are no exception.
