I have done many of those filters and they are way up on top, so I do not see a way for cabin air to recirclate back up to that point....the next generation of E320 [ 210 chassis] did away with them and actually put them in the cabin [ under glove box area] so that they would still filter cabin air regardless of recirc or not ..they also added a charcoal filter , which yours does not have ...yours are simply paper intake dust air filters .
If you take them out . see of there is a flapper there for recir., but I don't remember ever seeing any ....
There is a DIYer tutorial in the archieves for changing them...not a bad job once you do one...I suspect yours are clogged b/c every one I ever did was full of dirt and leaves. etc. ....prolly due to their under hood location ...not a real good set-up compared to the later chassis.
Like I said, may not be the filters, but I would definately look there First.....
Sure would be a lot of work if you dive into that dash only to find out later it was just restricted intake air b/c of a couple of dirty filters.....ya kno' ??

Again, is that 904????

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A Dalton