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.