No heat

I have a 2000 s10 with no heat. There is actually heat for about 5 minutes and then it just goes straight cold no matter how long I am driving. The temperature is about 150 degrees constantly. There is plenty of flow but I don't really like cold air when it's 7 degrees outside. If i turn the heat off and put it on 1 and on defrost the defrost stays warm but not enough to heat my truck. If you put it on vent it goes cold and if you put it on 2,3,or 4 it goes cold. I don't know what's wrong and could use some help.

You need to get the tempt up to 195 degrees F.
Check coolant level in rad and overfow tank.
Start with a good New thermostat.
Also if vents and defrost and heat do not switch good check for good engine vacuum to controls.
Auto or manual heater controls?
And let us know what it does.