I had posted a reply to the above but it doesn't seem to've appeared.
i used the loopback plug and everything worked, so it seems the bluetooth module is the culprit. so i will try and plug in the new one.
final question on this - the method i have seen online for plugging in the new module involves only connecting the power & ground cables of the new unit to the old unit, and then plugging the MOST cable into the new unit, so effectively continuing to use all the other wires in the blue plug of the old unit (see pic). is there any reason why i can't just plug the whole blue plug into the new unit??
thanks for your help.
Adrian