How can you tell when the charcoal filter has gone bad and needs replacement?

If you have maintenance records, you will know when was last changed , they recomend 12-15 k change or one year whichever happens first .
If you just bought the car and no record of last change, look at it and see if needs change , then that will be your start day , good luck

I looked under the glove compartment for the 2 that are supposed to be there, but only thing I see is a big piece of styrofoam and no compartment with air filters. Does anyone know if this means I don't have them or are they some where else?

If you have them , you should have a button on the dash board to activate the filters on/off .In the manual ,the button is called "Activated charcoal filter ". on the dash board , it is located left of the tempreture reading in the climate control screen .

On my C320, there are 2 charcoal filters under the glove box in addition to the charcoal filter over the battery. To change those, look under the glove box and you will see 2 torx screws - one on the left and one on the right. You have to kinda lay on the floor and look up to see them, but they would be closer to the knees of somebody sitting in the passenger seat. Once you find those, unscrew them and take them out. Now you can pull the cover down. It will catch on the left where there is an air duct close to the center console, but you can just wiggle it out. Once the cover is out of the car, lay on the floor again, look up with a flashlight and you will see a cover that says Open/Close with arrows. Follow the arrow to slide open the cover. The two filters are under that cover. They have these flexible tabs that you will bend down and simply pull each filter out. Put your new filters in, put the cover on while following the Close arrow to slide it shut, then put the big cover under the glove box and put the 2 torx screws back on. Very easy!

What year is your C320 ? My 2002 manual says that they are next to the front passenger right foot !!!

Mine is a 2003 C320, pretty much fully loaded. It is most certainly not next to the front passenger right foot.

Cabin Air Filter Picture in w203,w209

Wish i could open the picture . Do you have a DIY ? any help is appreciated .

U Can open the picture,just press on it.
To reach those air filter,you first have to remove a plastic kick panel or cover in order to access this flat panel,then two screws and the panel you see when you look under the dash comes out, it's underneath that. If I remember correctly the flat panel that the filters are behind slides to access them

2002 c320 MB , Does the summer convenience widows/sun roof oppen with remote key work on any one's car ? mine did work one time , then i played with some buttons such as circulation, carbon button , air volume, auto AC , then windows / roof will not open by remote any more. I tried Sync as manual says and with 10 diffrent sets of instructions by searching on this site , no change . Any help will be appreciated .

Thank you Johnand , very helpful to a first time owner !!!. thanks again .

I just assumed (like an idiot) that all the filters were the same and bought one with my last load of parts from the US. If you have a RHD car, you need a RHD filter. I thought they were rectangular so it would be cheaper than having two parts for the same thing.

Anyway the LHD version will still install fine but of course it has to be upside down filtering the wrong way. Does anyone think this would cause any problems?

I changed mine Sunday thanks to the OP for his post. This morning, while driving my car had a weird smell. Might try the charcoal filter.

How come these filters, sitting right on top of the battery, do not send battery fumes inside the cabin?

There's a vent tube going somewhere under the fender. I was also surprised