I don't know if this will help anyone but I recently had a problem on my daughter's 2000 Blazer. She would be driving and the vehical would suddenly shut off. The headlights wouldn't work, the car wouldn't turn over, and we we getting 12 volts on both sides of several relay solinoids in the under-hood fuse panel. After much searching I finally found that there was a loose ground on the rear of on of the head on her 4.3LTR engine. Once this was tightened the problems went away. The problem finally made sense to me as I had recently done a complete head gasket job on this engine and appearantely hadn't gotten this ground tight enough when it was put back together. I hope this info helps somone out there that may have a similar problem.