It's a little black container towards the front of the engine on the driver's side with a built-in dipstick in the cap. Why do you want to top it off? If its not leaking, the fluid level in there should be correct. Make sure you do not overfill the container.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record