On a '94 it could likely be a tired coolant thermostat as well. Shows up exactly in this kind of weather
Common thermostat failure is it sticks partly open constantly making for ages to the engine reaching normal working temp.
A simple test is a hand on the large hose running from engine into top of radiator. From cold engine start the hose should remain cold then rather abruptly raise to the scalding point after some 5min's.
I.e. if the hose instead slowly warms up and this goes on for ages - the thermostat is defective.
Ive read before that the heater control sometimes kinks and stops temperature control.I am just getting very slightly "aired" thru' the demister vents but not enough to clear the screen even after 10mins.I've checked all the obvious. ie heater hoses get hot on flow and return etc. but it looks a bit restricted to actually get into the actual heater box area.Any suggestions please. ( struggling to get seat belt on cos' of all the woolies) 
