Regarding including a co-signed car debt when filing chapter 13. Do I have to include the debt in my repayment plan?

I am assuming the $8,000 is an unsecured deficiency. The debt needs to be listed on your bankruptcy schedules to ensure it is discharged. If the bank files a proof of claim, it would normally share in the amount you propose to pay unsecured creditors. If they do not file a proof of claim timely, they do not get paid. If you make all plan payments as proposed, the debt will be discharged either way.

In most cases, your monthly payment and terms will be the same with or without the debt. In other words, unless there are unusual circumstances you will pay the same amount per month so you should list the debt.