Quite likely your resistor is partial broken. It is usually a little rectangular ceramic looking unit inside or close to your blower motor in the blower housing, sometimes they are a little tricky to find. More than likely it will look cracked but is still holding together at this point, it can only get a little worse (as in completely break and no blower). They just wear out under normal use in modern vehicles and frequently need to be replaced from anywheres from about 4-7 years, at least that's what it's been in my vehicles.
The motor which blows air through the vents.
A car heater taps off engine heat. If you aren't driving, then the engine is cold and there's no heat for the car heater to use.
An automobile's heater that blows only cold air could either be a faulty thermostat or, more likely, a faulty heater core. A trip to a mechanic or shop that specializes in radiator and cooling system repair should be able to diagnose and repair any problems.
Only blows cold air with the a/c on while driving, your cooling fan or fan clutch is bad depending on the year, make and model.......... Only blows cold air with the heater on while driving, you will have a bad or broken internal a/c - heater control door.....
The heater is like a small radiator(heater core) usually located in the dash. The fan blows air through it and heats the car. There are two heater hoses that carry coolant to & from the heater core.
Try your thermostat
The blower motor speed resistor is burned out.
The first thing that you need to check will be the coolant level..................
Although my car is not an Eclipse, I experienced the same issue. A mechanic determined that my radiator was partially blocked and recommended replacement. My heater now blows heat whether I'm stopped at a light or moving.
check your thermostat, if that isn't it you might need a new heater core. had one put in my f150 for about $150. With my car, it was Heater matrix clogged with radiator sealant. The workshop cleaned it and it's start working fine!
Because the resistor between the heater swithch and the heater motor is not functioning. Usually mounted on, in or near the heater enclosure. Its a common repair, and can be done by the do-it-yourselfer.
[first time answering questions] car heater works like the reverse of the normal cooling radiator - in fact the heater is like a small version of the rad that's on the frnt of the car - the hot water from the engine block goes into this small rad and the heater fan blows air thru it into the car - the hot heater rad loses heat into the car and - bingo - you get warm inside