Can anyone tell me where the cabin filter is on my c-class 203, I have had a look under the glove compartment but have not found anything that looks like a housing for a filter.

 

Under the hood, in the panel over the battery.

 

If you are asking about the activated charcoal filter (optional), It is located under the passenger side of the dash. If you are asking about the dust filter, then Laterthatsameday is right about it being over the battery. But also take note that there are 2 different types of filters installed over the battery.

If you have an activated charcoal filter, your filter above the battery will be the standard paper type filter.
If you don't have an activated charcoal filter, you will have a combination filter above the battery which has a carbon layer built into the filter along with the paper.

 

THANK YOU BOTH!!