How to ground PDC?

About the only thing I'm aware of in the PDC that is "ground" is the ground side of the relay coils. That, really should be pretty non critical. Grounding for EFI and electronics can be a real PITA. This gets worse with stuff like trunk mount batteries. If you have a chopped up adapted system, I've no idea how to get you started You might do some research on some of the Holley or Megasquirt documents for generalities on this