Great idea Larry! The air temp senor for interior air in the w126 is overhead above the rear view miror in the light console. There is what looks like a small plastic grill. Interior air should be flowing into this area.
Follow this link to a great article on MB climate control systems. http://www.mbca.org/MBCA_acc_system.htm
Good Luck, you'll be needing that AC for sure right about now.