That's a gooder in a wagon. My 98 E300 sedan has the "Bose" emblem on the rear deck speakers. Does you wagon have the emblem on any speaker?

If the wagon came with a sunroof, it's possible that it has Bose via the "E2" package when new. The E2 package was simply sunroof and Bose system combined. Not solid answers but somewhere to start?

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Barker