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If what you mean to say is that the vehicle starts only when there is a scan tool plugged in the DLC. Then the answer is a bad ground. You see, terminals 4 and 5 on the DLC are supposed to be ground (see graphic) and are bridging through the scan tool. To confirm, on top of the existing negative cable, run a couple of big wires from battery negative post; one to the motor, one to the body. If the engine still won't start, run an additional jumper directly first from terminal 4, and if the engine still won't start from terminal 5, to battery negative.
All the other stuff, such as the alarm acting up, etc. might get strightened up if also caused by bad ground.
Please let me know what you find.
Happy to hear you got the wife's Voyager up and running again!
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