One simply item to check are the flexible rubber connector tubes under the hood, near the fuse box/firewall area.

I helped a friend whose 190e had heat coming out the outside vents I believe while AC out the center.

His were cracked. Simple replacement.

You may want to try a search. There are many postings on the topic.

Haasman

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)