Voluntary Car Payments

My Chapter 7 bankruptcy was discharged in January and I did not sign a reaffirmation agreement for my car which is $1200 from being paid in full. I have been making voluntary payments but have been have unable to work due to medical problems therefore have had no income.

GMAC has told me that if I don’t continue to make payments that they will not take the car but the debt will be written off and will put a negative mark on my credit. Can they do this considering this car was included in the bankruptcy and the payments I have been making have been voluntary?