The module perhaps? clearly they can click, but don't when they're all plugged in - so its not down to you not being able to hear the click.

If they are not all plugged in, I am not surprised that the rears won't work - perhaps the problem is with the front PDC harness if its not the module?

An indy can scan it for much cheaper than BMW, my local one does it for free usually. Otherwise you are probably going to be pissing in the wind buying new parts to replace parts that may not be faulty.