A basic tenet of troubleshooting is determining when the problem started. If the plane has been flying for over ten years and the location of the antenna hasn't caused any issues, it's likely not that. Well, unless something has changed that would affect that (e.g. painting, covering, etc.).
Can you hit up one of your flying buddies to borrow an identical antenna for a temporary swap? Manufacturers don't exactly give those things away, so I wouldn't just buy a new one without knowing it's really a problem. How about the connectors...any corrosion on those? Is the cable itself in good shape? If in doubt, installing a new cable is a lot cheaper than an antenna.
Lastly, Garmin has a test where you measure the voltage at the antenna end of the cable--if it doesn't have 5 volts, then there's something wrong before the antenna. One of the Garmin experts may want to weigh in on that, as I'm going back a few years from memory. And that can be an iffy thing.