Hi, and welcome to Just Answer!
Assuming your engine is full of coolant, and coming up to temp (in the 190-200 range) then that is a fairly good indicator that your heater core is getting plugged up.
A quick test is after the car is warmed up feel both heater hoses going through the firewall, they should both be very hot.. if one is substantially cooler, that indicates that the heater core is restricted.
You can try flushing it out, just remove both hoses from the firewall (or wherever is easiest) and flush it both directions with a water hose. Sometimes this will cure the problem by blowing the crud out, once in a great while flushing them out will cause them to start leaking, but your facing replacement anyway if you want heat, so it's worth a shot.
I hope this helps, if you have any questions don't hesitate to reply back!
The heater core gets full coolant flow all the time.. The air is either directed through the heater core, or around it by the blend air door inside the heaterbox.
Generally when they start to get plugged up on the lowest fan setting you may get luke warm at best, XXXXX XXXXX you turn the fan speed up there isn't enough hot water circulating through the core to heat the larger volume of air.
It could be an issue with the blend air door, but the fact that it's warm until you crank the fan up doesn't necessarily support this.
Have you checked to see if both heater hoses get hot?
If both heater hoses are good an hot when the engine is warmed up, that's a pretty decent indicator that you've got flow through the heater core... hmm..
Let me look up the blend air door operation quick (some are cable, some are electric).. Ok it appears to be cable operated..
Try turning the temperature knob full to cold and hot, if you listen carefully you should be able to hear a soft 'thump' inside the box as the door goes to full travel..
If you do reply back, if you don't gently remove the knob (should be able to pull it straight toward you) and try turning the post with pliers and see if you get hot air. I have seen a few where the back of the knob strips out, it appears to work, but it won't actually turn the post..
Let me know,