Can someone help me out with the part number for a W202 C230?

I need a quick fix and will go to the dealer in the morning and would like to know the part number ahead of time. What I have is a W202 without the headlight lamp cleaning system, so my windshield washer bottle reservoir has a stopper to plug that hole. Well the stopper is no more so I need a a replacement.

Has anyone come up with a good jury rig for this, or do I need the exact fit from the factory part? So I look cool at the parts counter, can someone hook me with the part number?

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Michael

1988 300 SL (5 Speed)
1994 E320 Wagon
1997 C230