912iS Battery Discharge.

Oh boy...you are in real trouble when you have someone working on the harness that has no knowledge of the system.  White wire are because they are aircraft Tefcel (150C rated) wires and not junk automotive wire.  If you can't understand the installation manual then you should not ever cut or splice any of that harness.  All I can say is best of luck, I would not know where to begin has someone hacked into that main harness.  

The engine has a split generator stator.  It starts on the large stator side (B) and at 2500 RPM it will excite the small side, (A).  A runs all the engine critical parts B after that point is to run the bus for your aircraft.  The pin out is shown in the installation manual and you can find additional information in the maintenance manual heavy.  

Cheers

This is a full copy of the technical diagrams for the wires.  The installation manual, IM, has all the relevant ones for correct installation.  You can download it at www.flyrotax.com 

Not sure I agree with your A&P. White wires are standard in Aircraft very very surprised your A&P does not know this. We have many Tecnams on the field they do not seem to have higher rate of issues with wiring cf others.

Some A&P are adverse to any wiring especially if the grew up with old GA aircraft which have virtually none !

Modern aircraft generally have a wiring harness which allows the users to add in other items as they wish.

The IS engine ...of course requires much more wiring it is more like a modern car engine, a lot of sensors, circuits and busses if you A&P does not have experience with this type of system then you might need to find another or send them on an appropriate Rotax course. Both Rotax and Tecnam are professional companies and know what they are doing generally if you THINK they are wrong actually it is a lack of understanding about the systems they are infact almost always correct !

Anyway just my experience