Did you do a pressure test before bringing the car to the shop? Easy to see if the heater is letting warm air in, just feel the heater hoses in/out the firewall. If the hoses are cool, no heat is getting into the cabin.

My Behr made in China receiver dryer which I bought approx 5 years ago does not even have a sight glass.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked