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Most likely the GPS antenna. Very common in the sedan model, odd that is common in the wagon too even if the antenna should be a bit more protected against weather.
There was an antenna connector on the sedan that could pick up moisture. I think the wagon has this connector too but not sure about the position.
Check splitter (corrosion, etc.); eject/check condition of map data disc in navigator processor; remove/disassemble navigator processor to clean laser/eye with alcohol swab; check GPS antenna.