Weak Heat in the Winter

Since I purchased it, my 2004 Mazda 6 with V6 and man. trans has never produced really hot air from the heater in the winter. I max the auto temp to 90 degrees, but it never gets really hot. When pegged at the hottest setting, my wife's Ford Windstar with a manuel climate control is much, much hotter. My old Honda accord with a manual climate control was much hotter.

Is it just the way auto climate control systems work? Could I swap out a higher temp thermostat? Coolant levels indicate full.