2002 ford explorer heat issue

my 2002 ford explorer has had heat issues
over the last few years. I have had the blend
door repaired.. changed out the thermostat..
blew out heater core lines but im still noticing
a lag in heat. The heat works fine when the
outside temp is above 32f (takes a bit to
warm up when its cooler). But when its really
really cold it does not heat up right... doesnt
matter how long I let the vehicle run to warm
up. I am still chipping ice and sweeping snow
off windows and freezing in the vehicle as it
blows this semi warm air at you and it just
makes it feel colder... at a loss of what else to
look into.  Its older with higher miles but it
still runs well besides the heat issue. I'm
determined to run it til it dies but I am
dreading yet another mt winter in this vehicle.
So any suggestions on what to check.. there
is no clicking or sounds of struggle when I
switch from hot to cold or vice versa.  Ac
works just fine minus the god awful clicking it
makes when I engage it.. so I generally don't
run the ac.