Check to see if the engine is the same year as the car.....that would change the glow plug types as well.....if someone changed out the motor to a newer one, and never swapped over the glow plug wiring it may be that the previous owner had this problem all along and neber corrected it.

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00' C230K
88' 190E 2.3
79' 280CE (Sold)
84' 300SD
73' 450SE
93' 300E
68' VW Bug (what the hell was I thinking!)
99' HD FLSTF