I would tell them you're looking into filing Chapter 13 bankruptcy to consolidate all your debt. They'll know that since the vehicle is gone, this balance wouldn't be considered a secured/priority debt. I would elaborate that you might be able to avoid filing if you're able to get all of your creditors on the same page... Then offer to settle for 10% of the balance... they'll probably come back and offer to settle for 75%. This game goes back and forth, they might give a final amount that they'll need in order to break even. Let them know that's too much and it'll be cheaper to file bankrupt.
Almost all debt collectors have quotas and earn commission and incentives based on dollars collected. They want to collect the absolute most TODAY. Your goal is to pay the least, get it settled/taken care of so they won't sue you for the full balance. If you aren't getting anywhere with the collector, you can always call back or wait until they call you again. The longer your account ages they more likely they'll settle for less. They know they won't get much of anything if you file bankrupt.
NOTE: For this to be effective, DON'T TELL THEM you've already obtained an attorney. Then they won't be able to negotiate with you.
