Alternate ground strap location

[Cherjeff]

Hello. I have a 1995 1/2 gmc sierra k2500. I have intermittent ABS & Brake lights in dash. I also have 4wd not functioning. I also have malfunctioning heater. On one schematic, it looked like they all go through ABS unit. ABS unit is sealed and unable to come apart to resolder connections (cannot unseal unit for reflow repair). Obtained used ABS unit. Tried to redo chassis ground. Supposed to be located under drivers door. That ground is non-existent. Was never there. Who knows where the alternate ground spots are for this vehicle. I'm adding ground strap at ABS unit, down to frame. Want to find original ground. Heater has all new components. Loses power at fuse box when put under load. Need help. Please.