I think the guess was that it was an overvoltage condition or just a
faulty
cap.




I have just two 75W subs in my car and don't really need a capactor but
would it be a good idea to get one anyway to help take some of the load
off
of the car electrical system?

A capacitor won't really take the load off the electrical system. Buying a
fresh battery would probably do more to take the load off the alternator
than the addition of a capacitor.