I have a Euro 190E 2.3-16 with a factory installed manual a/c system that has a horizontal slider control for the temperature of the air in the car. This slider is not a control for the temperature of the air coming out of the vents. It controls the interior temperature of the car, and it is not a calibrated scale. The point of the markings is to make it intuitive to the person wanting to have the a/c cool him or her off more or less which way to push the lever. The lever merely sets the compressor duty cycle, and I do not even think there is a feedback sensor in the system to make the cycling a result of the temperature changes in the car's interior.
If the temperature in the car is too cold, you can turn the fan down to 2 (I am not sure the a/c works on a fan setting of 1) and turn the slider control to the shorter blue marks. This will turn the a/c compressor cycle from more time per hour "on" to less time per hour "on" and push a reduced volume of cooled air into the car's interior.
In my car I can also turn the heat on at the same time, and in New England, when the humidity is high and the windows are fogging up in a rain storm this really helps dry out the air and clear the windows. However, if it is hot outside, and the heat is turned on at the same time as the a/c, the dehumidifying characteristics continue, but the compressor will stay on longer but the cooling capacity of the system will be reduced. I actually think the heating system of the car can overwhelm the a/c system's capacity to remove heat.
By the way, the relays on my car are behind a black plastic shield behind the battery (on the passenger side of the space in front of the firewall). The shield is held in place by a rubber gasket that seals water and other stuff from the windshield area out of the protected space behind the battery where these and other relays are located across the width of the car. In the area of the OVP relay there are two relays located one in front of the other, with the OVP relay with its clear plastic cover and fuse on top, next to them, closer to the center of the car. Further inboard is the electronic ignition control module. I would find it odd that Mercedes would have a totally different layout for components in your model. The a/c relay is a little black box and has the word "KLIMA" on it, along with things like "12V" and "4 Zyl" in white letters. The other relay is an identical size black box and has "4 Zyl. 7100min" on it and I believe that is the fuel pump relay. The OVP relay is silver and is significantly smaller than the other two.
I hope this clarifies somethings for you.
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)