You're simply going to have to take your chance with the judge. You can ask that the car payment and insurance be included in your CS. That is, it's possible for the order to say "CS set at $1,000 per month paid to CP plus car payment and insurance. When car is paid off, CS goes to $1500 per month" or something like that.
If the court insists on paying ex all in cash, then I would ask for an order that she refinance the car in her own name or sell it to avoid damage to your credit rating. Also, if your name is not on the title, you'll want to remove that at the same time.
I really doubt that she'll get CS raised to $1500 PLUS the car and insurance if they're not covered in the decree. That is, if they're not listed under the property division section, then it's a completely voluntary payment in lieu of more CS, so I really don't see her double dipping. The court is going to want you to pay the guideline amount - and any extra gifts would be up to you.