I suspect I have the BM54 Back OG in my 2002/06 M3 as well, which is a shame as I'd like the Aux in option..

So what your saying is
1. find a toasted BM54NG,
2. Do some disassembley/solder/assembly work
3. Flash the modified BM54 Back OG
...
Profit?

More the point what harness work will I need to do? I want the Aux plug up near the driver as opposed to in the boot.

And how hard are bad BM54 NG radio modules to come by?