As tfabris said, check your extension cable. Also, to confirm, the 8-way connector on the empeg sled is wired like this: (looking into the wire-end of the connector with the latch at the top):

blue, purple, red, brown
black, no wire, grey, pink

n6mod had the "bad extension" problem, and AM was ok-ish (but had alternator whine and was quiet) and FM sounded AWFUL and VERY quiet. With a correctly wired cable everything was back to how it should be. If you're wondering how the FM tuner should sound with a good station, it should sound *excellent*. At least as good as any other tuner you've heard :)

You may need antenna power, depending on your car. What is it? Some cars have antenna power on the DIN connector.

Hugo