I have a 1988 GMC Sierra 1500, that recently after running with no problems at all, won't turn over at all. We know for a fact the starter is good, and to be safe, we had the starter itself tested, and sure enough, its good. The battery that was in it was bought in late 2018, but out of curiosity, we bought a brand new battery and still nothing. Dash lights come on and the over head light works, brake lights work, headlights, etc. The only noticeable thing is when I turn the turn signal on, the dash lights and everything else start flickering and flashing. Jump starting the truck also doesn't help at all. I want to see if anyone knows what the problem may be before I give up and have it towed to a mechanic.