there is a funnel for the mercedes transmissions, you will find the number below, also depending on the model you will need an adapter tube, and the special mercedes dipstick to check the fluid, the dipstick is for all models with transmissions with the 722.6XX prefix.
funnel- 126-589-12-63-00
extension tube- 140-589-49-63-00
dipstick- 140-589-15-21-00

best i can remember the funnel was pretty expensive.