1999 Cadillac STS car wont turn over

Your battery is under the seat, do you see where the ground cable connects, to the body. Assuming that your battery is good, and your connections are clean and tight, the next place to check is where the body grounds to the engine to complete the circuit. Check the large black cable from the body at the passeenger front corner of engine, to an alternator mounting bolt. I have seen these cables corrode and break. The next suspect would be checking for 12 volts at the starter. If you have a jumper cable, try clamping one of the upper strut mounting bolts, and bridging to a good engine grounding point. See if it starts, if so, you have a bad ground circuit.