2004 Volkswagen Beetle radio/antenna connection

A little over a year ago my father ordered a sirius radio to be installed in my car. It's got bluetooth, usb hookup, navigation, the whole nine yards. The only problem is the only place that could install it around us was best buy so installation guy didn't really know what he was doing and we had to go back once for missing microphone and other missing parts that he misplaced. After the installation I realized that my radio was constantly fuzzy and has horrible reception to most radio stations with the exception of one (which is polka.). I am assuming right now that my problem lies with the antenna but I'm not sure if I need to replace it or something is corroded. I'm just not sure what I'm supposed to be looking for!