weird problem...

[firehawk]

Could also be that your heater matrix is leaking coolant and releasing glycol fume to the inside of the car through the vents. Gycol tends to create an oily film on the inside of the windows and trying to wipe if off just smudges it around somewhat. Even a very small leak not easily visible when checking the coolant level on the radiator may cause this.

One way to diagnose this would be to override the heater matrix as whole, rerouting the coolant directly from one hose to another in the engine bay but those hoses are in a rather awkward place on the firewall behind the intake manifold.