Dr Ritchie Marcus (Battlefield Line DMU Group) has sent on some scans of the schematic wiring diagrams. They are identified for Class 117s, but the principles and circuit numbers should be the same for all blue square units.

Driver Controls Part 1 Part 2
Engine & Transmission Controls Part 1 Part 2
Fire Circuit Part 1 Part 2
Power Supplies Power Car Centre Car
Starting Circuit Part 1- unmodified Part 1 - Water Level Switch Modification Part 2 Start Relay Box
Lighting Circuits DMBS - Part 1 Part 2 DMS - Part 1 Part 2 Centre Car
Heating Circuits