Just my theory -- make money. Newer MBs tend to have drastically more expensive parts it seems. Currently, I'm testing if glow plug relay for 1985 300D is same as 1993 W124 2.5T . I heard the 1985 one works. You get it with 4 glow plugs for around 100.00. But you the W124 relay is around 150.00 on its own.

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius