The feds imposed it in 1996 but I do believe mercedes started using the obd2 connector under the dash sooner in certain model cars in U.S. only. I have a 96 SL500 and it is the first year of federal mandated obd2 compliance it has both the obd2 connector under the dash ,and the 38 pin connector under the hood on the passenger side in the electronics box.

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg