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It sounds like your first problem with the PO301 has been resolved. I don't know what took them so long. Plugs and wires are usually the first suspect.
What you have now appears to be a problem with your aftermarket alarm. Unfortunately these are all different brands and it's hard to tell you how to resolve it. They are designed to "interupt" certain curcuits so you can't just remove it, you have to splice the circuits back together. You may be able to recognize these wires with matching colors but you could cause more problems than you have now. The module is usually under the dash, to the left of the steering column. The best choice may be to take it to an alarm shop and let them remove it.
If the alarm is armed, it will disable the car. That's what it was put there for.
Possibly it's just a dead battery. Have you tried jumping it with another car? Make sure the cables are hooked up right if you do try.
You still need to have your battery and charging system tested along with checking for a draw. This is usually all combined into one test called a VAT test. That will tell you what is working and what isn't and whether it's safe to drive. It usually costs under $20.
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