Might be the heater water valve also.
There is a hose under the hood, attached to the firewall with a valve. If you have hot water on only one (or neither) side of this valve with the heat on: then it is quite possible the valve is stuck.
You can remove the three screws and then remove the double worm gear plastic gizmo; if the motor spins then the valve is hung up. If it does not move... then maybe it is the relay or something else. Caution: it will require patience and common sense to get it back together correctly.
Mine did this; and is still doing this. I simply removed the gizmo's, and opened the valve half way; haven't found any downside to doing this yet; seems there is an air bypass/shutoff for the heater core anyway. Oh well, anyway, just something to check.
I logged on to find my old thread about this, so i could order a new valve; if i find that thread i might remember come back and add the link to it.