I have some occassional steaming of my windshield and slight coolant smell in the cab...suspect heater core is bad. Removed glove box door and looked around and can see some green residue in the area. Not much water leakage in the cab. Very intermittant. I can see the inlet hoses for the core and the box it resides in. Is it really that big of a job to replace? Looks like I can do it right thru the glovebox. Could this be a clogged drain line and not the heater core? I drive a lot; been having bad headaches -wondering if this might be the cause. Any useful input would be appreciated; never found a job I couldn't do but I've read some horror stories regarding heater core replacement. Does the A/C come into play?