When you go to rent the car, you'll have the option to purchase a collision and physical damage waiver (zero deductible) and/or to buy liability insurance. If you have both comprehensive and collision coverage on your Mustang, then you may elect to not take the collision damage waiver, but if anything happens to the car, then you're on the hook for your policy's normal deductible for the Mustang. If you don't currently carry comp/collision on your Mustang, I would recommend adding it to your policy the day before you pick up the rental (and/or reducing the deductible REAL low to like 100 or 250 even if you do currently carry comp/coll on the 'Stang).. and then either crank it back up to where ever you have it now (or removing it again, if you don't currently carry it) once you turn in the rental. Don't tell your insurance company your intentions to reduce the coverage again after the weekend, nor explain why the change in the first place outside of "I'd like to". The small net-cost of doing this little trick is usually only a few bucks, as opposed to $15-20/day at the rental shop.

Anyhow. Hope that made sense.