strong collant smell when heater is on HELP?

i have a 2001 sport trac, and as i was driving at about 95 mph i started to smell collant, i replaced the thermostat about 5 months ago and everything was ok. now when i drive i smell this horrible stinch in the cab and when i role the windows down to disapate it, it gets worse. no oily discharge on window (yet)  and if i leave the truck and come back hours later the smell is still there. what is it? possibilites, thermostat, heater coil, head gasket? and i also checked resvoir after driving approx 50 miles, still full , very cool, and dark green as if mixed with slight oil. please any suggestion!

Sounds like you have a leaking heater core.

Hi check to see if the rug on passenger side front is damp because it could be the heater core and it does not always get up on the window. Hope this helps good luck.