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See if there is a small alarm speaker on the Right side engine compartment close to the ABS brake assembly. If you have speaker, look behind the glove box for the Keyless Entry-Anti-Theft module. There is a very high probability you have factory system installed.
 

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The dome lamp, luggage compartment lamp and courtesy lamps are powered all time. When a door is opened the door switch grounds the return side of the lamp and the circuit is complete. That is why the point your monitoring is going from +12 VDC to +2 VDC when you open the door, it would go to zero VDC but the ground connection is not that good. Your alarm system is designed to use this configuration (0 VDC on Open), the signal is called (-) door trigger input zone 3. If the vehicle used +12 on Open you would use the (+) door trigger input zone 3 Refer to page 16 and 17 of the installation manual.

I would not recommend ripping out the existing system unless you have a schematic available to see what operates what or you could create a situation where you disable the starting circuit.
 

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The door switch is using the body sheet metal as the return path back to the battery. There are a few ground straps between that point and the negative side of the battery, so there is some resistance in the circuit. I am sure your alarm system does not expect a zero value as the low level input, probably less than 5 VDC for a low level input or greater than 5 VDC for a high level input.
 
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