Most of these problems are vacuum leaks under the hood.
One hose on the reservoir. Disconnect the hose, is there vacuum on it? Plug it, see if your heater controls work.
Removing the panel looks something like this 94 pic. Vacuum lines are larger colored in the center. Black vacuum line disappearing into the shadows is the supply line which runs out through the firewall to the tee fitting, then to the vacuum reservoir.
If you need to remove the control panel, do not disconnect the lines from the panel. There's a quick-disconnect plug, make sure it's fully seated. Mine came from the factory with one side not clipped.