500SEL,Hot air mixed with cold blows from air conditioning

When my car has been sitting in the sun and is hot, instead of cold air coming from the air conditioning, I get hot air mixed with the cold from the two side air conditioner registers. When this happens, I have to shut down the two registers and switch on the extra air blower in order to divert the cold air to the center registers. The a/c duct in the back seat has no problem and it produces cold air at all times. I live in a hot climate where we have hot weather year round and I want to bypass all the hot water that goes to the cabin. How can I do this?