2000 Other Nissan Models interanl heater in Nissan Micra 00

Heater problem
2000 Other Nissan Models Engine Size unknown Front Wheel Drive 80000 miles
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Hi,
Ive got a Nissan Micra 2000, and the internal heater is not working, it takes about 15-20 mins just to get it warm, but never hot, im finding it hard to keep the windscreen lear of condensation,
Heres what ive tried;
1. check the cables from the knobs down to the cold air / hot air junction and everything is working and moving accordingly.
2. ive changed the thermostat but that doesnt seem to the problem as the old one looks perfect.

i think the problem is i dont really understand the internal working of the heater, i think its a little seperate radiator that runs off the engine radiator when it gets hot, my only suggestion would be probably that the little radiator has an air lock or something similar,
if you can advise me on what you think it might be all help is much appreciated, my next step would be booking it into a garage but im sure its something i could fix myself and fully compensate your site via "donation" for all useful info

thanks for your time

Keith