why does the vents smell in my 2002 bmw 316i

does it have aircon?

it might need new gas

or have you driven somewhere smelly? past a farm or anything?

Given time, the AC evaporator emits a musty smell due to the growth of microorganisms in its humid/wet environment.
And the smell from within the car that circulates into the evaporator fins add up to the problem (or is the real problem).
There r various ways to deal with this. But simply masking the smell with a deodorant is the least effective way. l suggest that u bring it to an aircon shop that could either:
a) inject cleaner solution into the evaporator cavity (this is a foamy
compound) & then run the ac blower to clean the fins,
b) or, electronic cleaning with a special tool