If I'm following these threads correctly, the 97 model is famous for destroying the blend door actuator on the electronic models. If you have a factory non-a/c model, I've heard that they are cable-driven and aren't as prone to this particular problem. I believe there are pages on the Tech section that explain how to check and repair this problem if the door is to blame.
Otherwise, to check the heater core itself, simplest test is to check the tubes coming out of the firewall, if you haven't done so already. If both sides are roughly the same temperature (very warm to hot), your heater core is probably ok. As for your vent position selector, definitely check vacuum. The door is controlled by vacuum on all models as far as I'm aware. There should be a line set (vacuum hoses) attached to the back of the HVAC control head that you replaced. Check there to see if you have vacuum present. If not, check at the firewall. Might need a diagram from a service book or the VECI label under the hood to find out exactly which hose is which.