Does anyone know if the turn signal bulbs on the E46 3-series BMW will work in the Countryman?

They should! The countryman part# is 63 21 7 160 789 while the E46 part# is 63 21 7 160 790 (this bulb is also used in rear break lights of R50-56)

While the part # is different, both are 12v 21 w, and both seem to have the same mounting prongs and should be interchangeable.

I compared both bulbs as i had them in stock.

Lupe