The heat in my s10 works fine when the weather is warm, I can't get it to shut off, even when the controls are turned to off hot air blows in all summer long, but now that the weather is cold the heat will not work, it works fine when I go to work in the afternoon when it's 50 out but when I leave work and night I have no heat, one thing I've noticed is the feed hose to the heater core is hot but he return is cold, so I'm figuring coolant isn't flowing through the core, I don't think the core is clogged though because I can run water through it no problem, also I've noticed what sounds like running water behind the dash when I accelerate, when this happens he heat works briefly, any help would be greatly appreciated Thank you

Check the heater control valve. It's not uncommon for them to fail.