The heater valves on a 108 are behind the glove box as I recall. Rebuilding is fairly straightforward and self-evident once you see them.

The bolts with the cross-drilled holes that the wire to the heater levers are clamped in can be broken, so one needs to be somewhat careful with locking down the nut on the wire; don't go too far.

Rebuilding the heater control levers is a bit troublesome as it is not easy to remove or re-install the assy. Access to the two nuts that holds the bezel in and clamps the heater lever frame in is not easy, and once you get past that part, manipuating the assy out and back in with four stiff wires and cable housings attached is not the easiest job, but is definitely do-able.

You can scout it out by taking out the glove box, which is an easy task, and taking out the speaker grille and speaker and taking a look with a flashlight and mirror at the heater control assy and the cables going to the heater valves. Try to see the two nuts (one on each side) holding the heater control assy in; they're facing toward the engine.

Once you get the control assy out, rebuilding is easy. Get yourself a set of new heater levers. The one with the rheostat that controls the fan speed (the blue one) is usually a seperate item from the heater lever set and is the pricey one. What usually is the problem is broken or breaking of the rubber handles. If you're going to go to all the trouble of getting that thing out & back in, you'll want to make sure those levers are going to last a while.