Need more heat in Esteem

I have an '02 (1.8) and a '98 (1.6) and when it's cold outside both cars do not get warm inside. When I'm on the highway the heat gets colder but if it's just sitting around idling then it barely gets warm. Both cars have had the thermostat replaced and cooling system flushed and the sensors are fine. What seems to be the problem? Thanks

What does the temp gauge show during these times?
Does it sit at the half way mark between hot and cold (normal) or does it never move above the cold mark (not normal)?
If the temp gauge is in it's normal operating range (halfway), then the heater core is not getting coolant flow or the temp door (inside under the dash) is not closing properly. Also, the cooling system could be low on coolant.
If the temp gauge stays at the cold mark, then the thermostat is staying open.

Good luck!

My 99 Esteem 1.6 takes about 1.5 miles before I get a decent amount of heat, longer when it gets really cold out, are you driving yours further than this? I'm assuming so since you said "...on the highway...".

What did the old antifreeze look like when it came out?

Its weird that both of your cars are doing this, how cold is it getting where you live?