The I-Bus wire is white and red with some yellow. You should find it in the connectors to the Mk-III unit. You could use a voltmeter and check if it has a voltage of some 12V (maybe a bit less if there is I-Bus traffic).But to be honest I do not think the problem lays there. It would seem the unit can not read a CD, no matter if it may also not be able to communicate. I'm out of ideas now, really. Maybe the unit has hardware problem and maybe someone with more experience can help you further.