All 6 glow plug failure means that it us your glowplug control unit that is faulty. Unless all 6 have infact failed, but I'd eat my hat if they have! So, I had this problem with my 330d too not so long ago, but I'm buggered if I can point you to its location! It gave me a "misfire" effect on cold starts which obviously, I got mildly flamed for in my thread at the time in the N57 board, especially as I didn't clearly state my knowledge of homogeneous charge compression ignition and the lack of spark plugs :P. So, have a look, hope it helps. What are your symptoms, if any?
2006 BMW E60 520d SE 6MT
2009 BMW E90-LCI 330d SE 6MT
Current: 2012 F30 328i Luxury 8AT
2015 Audi A6 Avant Ultra, Black Edition (sorry!)