It may be a good idea to pour some water into the fresh air intake and see at what rate it drains through. I’ll try this sometime today on my 94 E320 and report back for comparative purposes. Also, check in the nooks & crannies near the battery and fuse box for composting leaves and other junk, it may be an indication as to what’s going on in the intake channels. Finally, the cabin filters may be so clogged that, effectively, the air is re-circulating all the time.