It could be the coax cable itself, especially if it's original. Moisture eventually gets in there and degrades performance. Personally, I'd just get an antenna and coax kit. They're not too expensive, it can be a pain in the butt to run the wire correctly (depending on how it's run), but can be done buy most people. If you just want to try a coax, make sure you get cable meant for AM/FM radios for best reception, which is 92 ohms impedance. You can hook it up through the door to see if it works before you pull things apart to run it neatly behind the dash, headliner or wherever it's run.