Does the dash dim at all, any lights? you may want a multimeter to check resistance and for voltage. Recheck the wires to the starter and the actual ground wire, any place for corrosion (including the ground wire/bolt on the thermostat casing). If this is all bad, could be the ignition switch. The relay should go click, hum for two seconds, and click again (the hum stops.) this is sending power to the fuel pump. After this, power goes to the signal wire for the starter. Recheck the 12mm nut at the starter motor to ensure its connected right. Also look around like the casing and ground strap for massive amounts of dirt or oil that could impede power.