348 wiring, ground point 150a/b whereabouts?

Hi, Some of you may have read another thread about my running issues. I've found that my ECU pin 10 on both sides don't drop to ground (one at 55ohms the other at 25ohms), so am trying to locate the physical location of the ground points 150a/b to correct. I'd appreciate any assistance in finding where this location is?

If I can't find this point then I'll have to stick in an additional ground direct to pin 10, but don't really want to break into the wiring if I can avoid it. i'd rather correct the issue properly.

 

No clues out there?