The truck obviously was not properly taken care of by the previous owner. When I sold my 1984 280GE in 2002 with 111,000 miles, it still ran like new.

You need to get an EPC for the truck to help you determine the correct part numbers. You then have at least three sources in North America that I know of to get what you need. And you are correct, NA MB dealers, on the whole, don't have a clue when it comes to the G - even the ones (2002-2005) they're selling now.

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1973 280SE 4.5 (Sold)
1984 LWB 280GE (Sold)
2000 G500 NMLE - "Deep Throat"
2007 ML320 CDI (Sold)
2010 Porsche Cayenne S Transsyberia TipTronic (Sold)
2010 Audi Q7 3.0 TDI (Sold and Sorely Missed)
2014 ML350 BTC
2015 Audi Q7 3.0 TDI (On Order)