With regards to the heater flaps, the most common problem is that they aren't operating across the full range so while you are happily moving the levers on the dash up / down you are actually only moving the flaps partway (if at all). Usually this is just down to sticky cables / poor adjustment and can be resolved in a matter of minutes.
Easiest way to check is to disconnect the operating cables from the flaps and move the flaps by hand noting the full operating range. That done, re-connect the cables and operate the levers and see if the flaps are moving the same amount, if not adjustment is required. While you are at it make sure that the cable inner moves freely and the outer is firmly clamped as it is very common for them to become seized and move together resulting in no movement at the flaps.
In total you have three lever controls but only two of them connect to the heater itself (the air distribution lever works on flaps in the lower dash).
The cable that is connected to the heater ON / OFF lever (L/H side) operates a flap that closes against the bulkhead. As you operate the lever, the flap opens allowing air through into he lower dash and as the lever gets lower it operates the fan motor (2-speed), If this flap isn't opening fully then air can't get through into the cab whereas on the other hand, if it isn't closing fully then it will allow a through-flow of air even if you have the heater off and don't want air in.
The temperature control lever (RH side) operates the flap that diverts air through or around the matrix. On HOT all air flows
through the matrix, on COLD all air is diverted
around the matrix and at any point in-between heated air is mixed with un-heated air. Simple but effective.
People have asked in the past where the water mixing valve is located but the simple answer is that there isn't one! The matrix runs at engine coolant temperature
all the time and then air into the cab is simply flows through or around it.
The other common problem is damaged seals (age related as the seal material gets hard and crumbles) but to get at those you need to remove & open the heater, no big deal but do the other checks first.
Covering the radiator is only something you need to do if the cooling system isn't working correctly in the first place.
Coolant isn't diverted through the radiator until the engine is up to operating temperature and the thermostat opens. The heater however is in the circuit that receives coolant at jacket temperature. This means that when the engine is warming up the top hose should remain cold while the heater hoses get warm / hot.
Hope that helps understand how it all works.
