There should be a toll free number on the card or contract; just call and cancel. If you are paying the car off, wait until it's paid and then cancel, or the refund will go to the lienholder. The refund will be prorated months or miles, minus a cancellation fee in some states. Usage doesn't matter. If it's expired you will get no refund, unless you have it in writing that you do. If so, check to make sure all the conditions are met (ie, the contract has gone full term, with no claims, not even a rental claim).
Mosca