I've got a 99' Chevy S-10. The heaters has slowly lost its ability to put out heat over the last 2 or 3 years. It only blows cold air now. The Engine temp only gets up to about 150 degrees. It got the original radiator. It can create heat when the weather is warmer. Im not sure what to make of the problem? Any ideas?

Try flushing the heater core; we've flushed quite a few of them recently.

In my opinion, and from past experience, it sounds like the thermostat. Try using this link for helpful ways to check it out.
How to Tell if Your Car's Thermostat Is Stuck Closed | eHow.com (http://www.ehow.com/how_7683_tell-cars-thermostat.html)