'03 Sebring Convertible Heater Problem
 

Not getting heat when car is warmed-up...temp guage doesn't rise to half-way point....coolant was low....put in Zerex G-05 (HOAT) to max level (cold)...opened bleeder valve, but only coolant coming out....my only other thought is the thermostat....car has only 33,000 miles......any thoughts

If you search the forum here you will find that for most people, the temp gauge spends most of its time below the mid-point and further, that you should never touch that bleed screw. The fact that you were actually able to open the screw without spinning the insert in the plastic leads me to suspect that the part was probably recently replaced. The coolant outlet manifold is usually sold with a temperature sensor and aftermarket sensors are often found to cause the gauge to read a bit low.
Anyway, if the coolant level is low, it could explain the poor heater performance. See if it improves now that you have filled the system. Slightly overfilling won't hurt anything.
If that doesn't help, then there are numerous other possible sources of the problem including the thermostat, plugged heater core, faulty blend door operation, bad control head, bad body computer, bad conections between the foregoing components.
My '01 convertible's heater conks out at unpredictable moments. For some reason, switching the fan to "0" for a few minutes then seems to get it going again. Who knows why? Having the thing operate through the body computer seems to have added a layer of unneccessary complexity that generates its own problems. Can't imagine why they felt it was required to design it that way.

This, is of course assuming that you don't have Automatic Temperature Control. That will adjust the temperatureof the discharge air to lessen as the interior of the car warms up.