As mentioned, typical culprit is the vacuum operated heater valve under the hood. If there is no vacuum at the valve, are all the heater doors opening and closing as they should? If not, check the vacuum at the reservoir under the hood. Check it first, since it is easy to check. All vacuum controlled devices are powered off it.

If the doors are working, then pull out the controls and check the vacuum actuator valve on the end of the temperature control lever. The water valve is controlled by it.

If it is the water valve, there are after market replacements as mentioned, or MegaParts also has correct appearing ones at a reasonable price.