2005 dodge magnum heater issue?

[darlingboltz]

I have a 2005 DODGE MAGNUM HEMI recently the heater stopped working during normal driving, however it will work at high speeds (such as freeway) but not local around town stuff... from reading online it sounds like maybe a resistor?
does anyone know what it might be, how to replace it, and where to get the part???...
Thanks so much!