Hi Mick,
The wiring may be a problem??? How are you with a meter and pliers.
I fitted a Cruise Control to an 06' TD4 Manual HSE (has Becker unit as std) - traced out much of the 'standard CC' wiring against the Rave and found that it's not there, what is there does not appear at the right connectors down by the tunnel (re. Rave) and there are different tunnel area wiring sub-looms listed in the Microcat for manual and auto for the same year. (only the auto's got the CC option in 06')
The CC uses the same road speed signal derived from the ABS CCU as the Audio head unit - if your audio unit varies volume with road speed then this is already present at the head unit (red/white or orange/white wire).
The direction feed, from memory, uses the reversing light signal - don't remember where that appears at head unit plugs.
Probably do-able but you may need to run in a few wires and fit some adaptors to match to the Becker DIN layout?
Hope my recollections help.
TD John