This response regarded a '97 E series, you '99 may be the same.
The radio uses an antenna preamplifier that's located (I read) behind the rear seat and left rear (driver's side) pillar upholstery (of sedan). The preamp is badly built, IMHO, as it uses CU rivets to ground its circuit board to an AL frame. Moisture causes electrolysis at that ground connection and a lousy reception results. Some engineering! The fix is to solder a (new, added) ground wire to a CU rivet and manually connect that ground wire to the body.