The mall cops (or whoever called the cops) are witnesses. That's enough evidence if he took thier statements. The cop would, probably, have to witness to arrest you. Since he didn't see/hear it he just wrote the ticket.
I'm guessing the cop looked in the car and saw your system and/or you admitted you were making noise? If so, that confirms the witnesses statements. Plenty of proof to convict you.
You could try fighting it, to get off on a techinicallity, but the fact is your guilty so the odds are not in your favor. You knew you were annoying the hell out of people there and you knew that the cops are cracking down on people who pound bass. You got caught.
Another strategy would be to show up to court and man up, tell him your wrote a letter to the mall apologizing for distrubing thier business and customers (give the judge a copy), tell them you didn't realize it could be heard so far away, you've torn out the system (take the sub box out of your car don't lie to the judge), and beg him to reduce the penalty to a non-misdemeanor because your already on probation and this could have very serious consequences for you. Tell him about a local group you volunteer with. If you don't, find one quick. Volunteer to help kids at your community center. Volunteer work with kids could really help.
If your already on probation, try to stay out of trouble instead of trying to get away with stuff.