The procedure is pretty well laid out in Bentley Section 1-10.1. See the "Dash" topic in INTERIORS forum where another member has discusses some issues he faced. The instrument panel will have to be removed for working room to replace the assembly.
The Bentley manual does go into detail about the control assembly removal and replacement, but what I couldn't find was detailed instruction on how to remove and replace the cables. I just got into and under my '78 to figure it out.
Here's what I did to my '78:
1. I have just the stock heater (heat supplied through the heater boxes only, no auxillary heat).Under the dash, on the control assembly, there are 4 cables attached. On mine, the left 2 control directional flaps and the right 2 open the heater control box flaps. They are held on with a spring clip. To remove a cable, this clip comes off.
2. The left heater box control cable has a "dogleg" that hooks onto the control assembly and the right cable hooks onto the dogleg and is held on by an "E" clip. Since I was only replacing the right one, I just removed the "E" clip and slid the cable off.
3. Remove the tray under the front of the bus. You will notice 3 cables (should be clipped together after coming through from inside to outside). I pushed a little of the cable out from the inside to find the right one. The cable enters a hard tube at this point.
4. Rear of bus: Loosen the barrel nut that holds the cable at the heater control box (my cable was broken at this end), remove the bellows, and remove the clip that holds the solid tube to the frame. There is a short piece that the bellows attaches to, which is at the end of the solid tube,held to the frame by the aformentioned clip...pull it off. When the tube is bent down a little, you can see there is a little plastic "plug" at the end of the solid tube with the cable coming through it. Pull it out and take it off the cable through a slit on it's side.
5. Back at the front of the bus, I proceeded to pull the cable out. There is an inner cable surrounded by a body (like a cable within a cable). The inner cable is the cable that hooks up at front and back. When the old cable was out, I sprayed a little graphite lubricant up into the solid tube and the new cable just slid in like butter.
6. After the cable was pushed all the way in to the rear, I fed the front end of the cable back up into the bus, hooked it back up (a small pain, but do-able) and re-assembled the parts taken off in removal and hooked up the cable to the heater control box. After making sure that the cable did it's job, I replaced the tray under the front. All done!
This cable had been broken for quite a while, and my '78 had not been heating the interior the way it should have. Now, it will burn me up just like my '79 does. I guess having both heater boxes doing their jobs makes all the difference in the world.
Oh, and by the way, I just did this today!
(I'm pretty sure I didn't forget a step when trying to remember what I did...if I did, I'll make a correction or addition)
There is a good description of the heat/vent system control cables in the Bentley, §80.3-8.10.
I'm not sure which the 2nd lever controls in an air-cooled. That should be in your owner's manual. In the later models, it controled the amount of heat into the cabin. If so, it controls the heater control flaps in the exhaust where hot air exits the heat exchangers. Full left is off, so moving pulls the lever against the spring to open the flap to the interior. Closed (off) the flap dumps the heat to ground.
The usual method to diagnose cable failures is to disconnect the cable at both ends. See if the control lever now moves freely. Then see if the controlled item or flaps move freely by hand. Then operate the cable -- still disconnected -- to see if it moves freely. This should isolate your problem area.
Cable have an inherant weakness. The are designed to PULL, but in many cases are asked to push. This is OK if there is a strong return spring, like in a clutch, but in something like the heat, they are asked to push with primarily the cable rigidity. This requires the cable sheath, mounts and unsupported end sections to keep the cable from bowing. Cables will take the least resistance and bending is often easier than operating the tight or damaged component.
I have a problem with the heater of my car since I bought it, last may.
Regardless the position of the temp control, the air coming out is always cold. I dont get any minimal variation in the air temp by turning it to the hot side.
Should it be a problem with the heater radiator, or is just a simple case of a temp control cable?