Almost no heat inside, just some lukewarm air coming through. Changed t-stat, engine temperature is always normal (although takes long to reach the normal operational temperature), hoses to the heater core both are warm and with the engine coolant temperature.... took off the hoses to the heater core and run water through it - nice and clean, not clogged at all... but still some lukewarm air coming through   . What to check/inspect?

Can we assume you're asking about a 98~05 Tracker/Vitara/Grand Vitara?

If so - the air temp is largely determined by "blend doors" in the plenum chamber - the temp lever (if you have an early model with horizontal levers) is connected via a bowden cable to a door, the position of which decides how much warm air is mixed into the air stream.  If you have the later model with a round knob, I believe that uses an electrical actuator to set the door position.

2002 Vitara.

Since everything is pointing pretty much to the blending door - is there a way to check it without dissembling the complete unit?