Heating problem

My smal pickup 1999 S-10 does not provide any heat whatsoever or should I say very little. Up here in Canada these days we are suffering a cold wave anywhere between -10 and -25° (celcius). Can anyone point my nose in the right direction as to where to look for this problem. I replaced the thermostat and the temprature gauge is giving me a reading of just below 200° (f). I've been told that there could be an aanother thermostat underneath the dash to check. Furthermore I mixed a cleaning product in the anti freeze liquid to clean the conduits and apparently I have to flush the system and re-fill the system with new anti-freeze. The cleaning product have been out in for alamost 3 days now  but I don't see any improvement so far. If any scale existed in the conduits, the heat should have come back. Thanks for your help. 

For those of you that are having problems with the heat and losing antifreeze, I discovered from my mechanic and from the web, that it is a problem with GM trucks and cars.  It has to do with the intake manifold, and yes the head gasket leaking.  There is a website, whereby GM is responsible for fixing this problem, under a Supreme Court Ruling.  They are able to file a claim for consideration of compensation.  We should do the same here and contact the Department of Justice with this information, collectively.

My problem is fixed. The system was flushed and did not work too well. Further checking by my  mechanic brought down the final diagnostic. The heater rad was totally blocked. Hardly any fluid circulation. Since this rad is made of copper, he used muriatic acid which he injected  in the rad and then flushed it again. You should have seen the gunk that came out. Now I know why they recommend flushing and replacing with the recommended anti-freeze at approx. 40K miles. Replacing the spark plugs at 50k. Also replacing the transmission oil, the power steering oil and the brake fluid is apparently a must if you want to avoid problems. For this one the cost came out at $150. My vehicle is 52k miles so all the other  things have got to be done. I have  a hefty "doctor's" appoitment sceduleto respect in the next month or so.