Had the same trouble with my
93 s-10 blazer. My trouble was with the
antenna connecter which was , on the passenger side,
under the hood , right under the blower motor. If you
follow the cable from the antenna, from the the fender ,it drops
down, under the blower motor, the connector, then heads through
the firewall then into the radio.
I think you might have the same setup.
Cut out the connector, solder the braid ground and the center
conductor, and VOLA! (sic) beautiful music, and your favorite
talk show on AM...In other words, get rid of that connector.
My 2 cents, it worked for me!