Merry Christmas all, im having a no start issue with my blazer, I've replaced the battery, starter spark plugs and wires, cleaned battery connections and starter connections with a wire brush, took off starter and tested to see if works, tested voltage at battery and from battery negative to alternator at 12.5 volts with vehicle off. I tried starting it in neutral, with the dash lights turned down and radio off, tried jump starting it with a booster and another vehicle, cant get it started. I have a fuel filter im going to put in, it has a newer maf sensor. It cranked decently when I first put the new battery and starter but now cranks very slow and seems like a bad battery although the batterys new, voltage seemed good on my check and I've tried jumping it. Im stumped and not sure where to go from here its been parked for 3 days now. Thanks for any input!

 

Ground strap maybe?