You could check the radio grounding strap. It's a short piece of braided wire that is bolted to the passenger side of the muffler. These tend to rot out over time. You could also check your antenna connections. You'll find them by pulling down the carpeting on the drivers side of the center console near the ashtray area. Here you should see two black wires...one for the power antenna, one for the rear window antenna. Make sure both are connected (this is also handy to know when you take out your stock radio...you can't pull out your radio without disconnecting these antenna leads)

Dave