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The truck has been broken down for maybe 6 months now, my wife still refuses to sell it to them, and it's just going to rot unless they continue to fix it themselves. Now I'm furious. I spend good money on that truck, took care of it, and my kid is being abused because of my ex's entitlement. I finally lost it last night and ranted to my kid when this came up- that should be my truck, I paid every penny on that truck, your mom's a selfish <beep beep>, etc.
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I agree with the advice others have given for you personally to stay out of it, however, you could suggest a bit of advice to your daughter.
Tell her mom that she either signs the title over to them, or they're going to have it towed, and your ex-would be stuck with the fees for impoundment and storage if she wants it back.
Had a family member in a situation like this, where they were driving a car another family member loaned them, but that person was not responding to questions about the insurance and registration fees (the family member had offered to pay them since they were using the car, but just couldn't get a response). They finally had to threaten to abandon the car on the freeway to stick them with the towing and impoundment fees, and then of course they got a response...
When faced with someone being difficult & unreasonable like this, I think you need to think of what leverage you have, and play hardball. They won't like it, but as I've come to realize, if your ex is BPD, they're not going to appreciate any kind gestures, and will bad mouth you either way, so you might as well get the best deal you can in any interaction with them.