Have you ever replaced the thermostat? You might have a thermostat that's stuck open. You can use the electronic odometer trick to monitor engine temp, and that should show you if your thermostat is working- it showed on mine. The temps are in C, so you'll have to familiarize yourself with conversion before you take the car for a spin. Look for the odo read out to show something like this [20 C], and that's the temp reading. The actual number will depend on the ambient temp and the engine temp at the time. If the thermostat was working, then you should see your engine temp run up to 95C or so, then drop down when the thermostat opens, run up to 95C again, and drop down until it settles right around the thermostat temp. The colder it is outside, the longer it will take to settle- if it ever does. If you don't see it heat up in cycles like that after 10 minutes of driving- then you definitely have a stuck thermostat. My car takes only 3 minutes to reach the point where it cycles in 15C (60F-ish) ambient temps.
Coolant flows through the heater core all the time in this car- there is no valve, so ignore anyone who says that there's a valve you need to replace. To check the heater core itself, drain coolant, remove the 2 heater core hoses from the engine, and then put a water hose on it. If there was anything in there- it should clean it out. I wouldn't expect that there was anything in there.
