An automatic has a
starter lockout when the transmission is not in P or N gear.
and Aftermarket alarms are a total unknown...
Normal troubleshooting (ignoring the aftermarket alarm):
1. Get a test light at the auto parts store, Harbor Freight or Walmart. You want the one that looks like an old-fashioned ice pick with a 12 volt bulb in a transparent handle, and a long ground wire with an alligator clip. Test the test light by hooking the alligator to a good body or engine ground, then touching the jump start terminal with the sharp point - the bulb should light up. You can use a meter, but the test light is better and easier.
2. Open the panel exposing the Central Electric Panel. Locate the plugs on the bottom, and figure out the naming convention - alpha from left to right, with no I (i) plug. The terminals on the plugs are arranged:
Use the color codes on the wires from the wiring diagrams as an additional aid.
3. Hook the alligator clip to one of the ground points above the panel. Touch the mani power points at the top of the panel - the bulb should light.
4. Find Terminal H21. A, B, C, D, E, F, G, H - 8TH plug. 21 is the bottom terminal on the right side. The wire is yellow. Touch the terminal with the sharp point and turn the key to the start position - the bulb should light. If not, either the ignition switch, its wiring, or your test are faulty. If you have power, all is well so far.
5. Find Terminal C12. A, B, C - 3rd plug. 12 is 2nd up on the left side, and the wire is yellow. Touch C12 and turn the key to the start position - the bulb should light. If not, the start relay is faulty, or the neutral safety switch is not making connection (not in Park?), the wiring is faulty, or your test is faulty. If you have power, all is well thus far in the circuit.
6. Go under the hood and find the 14-pin connector near the jump start terminal. Find the yellow wire on one corner. Hook the alligator to a good ground, touch the yellow wire terminal on one side of the connector and have someone turn the key to the start position - the bulb should light. If not, touch the yellow wire terminal on the other side of the connector - if you have power on one side but not the other, the connector is faulty or dirty. If you have power on both sides, the starter solenoid should make an audible "clack!" when the bulb lights. If not, the battery connections are the first suspect, especially the ground cables. If the battery cables are good to the starter, the solenoid may be faulty, but that is pretty rare.
Find where the power disappears on the yellow wire...