diagram showing thermostat

Follow the upper radiator hose to the thermostat housing behind the AC compressor. What's the symptom? Thermostats don't fail very often.

always runs cool and heat is very weak

Well, that does sound like a thermostat problem. I was hoping to avoid that because they can be kind of hard to get to. You might want to unbolt the AC compressor to give yourself better access.

Consider squeezing the hose first while the engine is idling to verify the temperature comes up. It could take a couple of minutes.