I have tried to adjust the heater cable in the roof several times but can't get it to work properly and taking the roof apart each time is ridiculous! It is ok in the winter time but not in the summer. There is no air conditioner but the flowing air from the fan is a help if the heat is off. I would like to put a valve in the heater line to shut it off in the summer if possible. I also know some systems require two valves- an in and out. Can this be done without wrecking something and where is the best place. Thanks for any help.
Maybe you could try this......cut a short section of hose & circumvent the system so that it just flows in a short loop under the hood. Ours is in the heater mode in this picture on our old 580D, but in the summer we just bypassed the heater under the hood. Just have to work fast when you do it so you don't loose much fluid!
Locate where the hot water source is on the engine. Most likely on the thermostat housing like Metalman's. Install a ball valve there . You will only need one valve to stop the flow to the heater.
There's a valve on my hoe on the hose that goes up to the heater core , that should be the only valve you need as Tinkerer said.