Click,Click,Click!

I have had grounds do this, bad cables do this and batteries also.
The tell tale is the power to everything else dropping out.
When i take the engine ground off i inspect it closely and look for evidence of arcing. Also, lock washers arent great for battery cables. No washer gives more surface area to pass current.
Clean bright and tight!
Glad its solved.
Test system after you make one change,this way you know what fixed it.