I bought my van used. It has had issues with starting regularly, the alarmdingin

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We have replaced the starter, battery, checked the alternator. It just doesn't start with no rhyme or reason whenever it wants, generally at least once a month and has to be jumped. In fact, this last time it wouldn't even jump. I think it's an electrical/computer issue, but no one wants to take me seriously. The alarm or dinging and message on the dash says rear door ajar or whatever, and no matter how many times I shut all the doors, again with no rhyme or reason it will stop or it won't.