The box is directly below the heater core. The heater/air flow levers operate the louvers (simple mechanical flaps attached to the cables operated by the levers)and are just about bullet proof. Your problem likely rests with the cables themselves, they probably need to be lubed. The crude hinges on the flaps can also be lubed. In the worst case the cable(s) were kinked during reassembly. In any case, all will be revealed when you remove the box and inspect the operation. There are several other posts on the removal process, it is a bit difficult... the bottom keeper bolt/nut go through the tunnel and you will probably need to fish a spanner from underneath the car, directly above the front transmission housing. Once released, the box slides right into th passenger footwell.
The other common problem is for the bracket on the side of the heater core to separate from the core itself, causing the cable to the valve to be ineffective (moving the cable then only moves the box, without fully-opening or closing the valve). If this is the case, it can be brazed on by any radiator shop. And, BTW, there is an access hole to release the cable to the heater valve located in the engine compartment/firewall. I learned about that the hard way :-)
Best,
g
'64 230sl, fully sorted out...ooops, spoke too soon
