Kalim found out that there are two blue plugs and more information about the MK60 vs the MK20 DSC system.
Looking in the ST034 pdf I find this for the MK60 DSC.
There are two types of sensors used in the rear axle of the E46:
• The short sensor is used on the 325i (any transmission) and 330i automatic.
• The long sensor is used on the 330i manual transmission version.
Doesn't say much about the M3 but if REALOEM is correct, it uses the short one like all other E46s except 330i manuals. :facepalm. Looks like I need to find some spacers to run temporarily till I can get a set of wheel sensors.
Grey Plug:
inductive sensors
Blue plug:
magnetoresistive sensors