heater core leaking?

[methodwares]

The last two times i've used the car (which has been driving to work), when i get in it in the evening the windscreen has been slightly misted on the inside. running the defrost does nothing to clear it, so i wipe it away and when i get home clean it off with vinegar water and newspaper, park it in the garage and it is still clean overnight.

this to me says the heater core is leaking and its coolant being sprayed up. I do have to top up the coolant once a week or so (about a liter max) but i know i have a very small leak on one of the bleed valves (not bad, but enough to leave a blue trace). But i never, ever smell coolant in the car and for the last 5 miles last night i ran the hot fan fully on (got a bit warm in the car) and there was no additional 'mist' on the window by the time i got home.

there are two things i have noticed, these two days the sun has been out here in manchester and it has been quite warm in the car as the sun shines on it all day long. the car was serviced (and valeted) by the boys at automedon, so could it be the stuff they cleaned the dash with (made it all nice and shiney) be evaporating and misting the wind screen? and the other, the carpet was damp through the winter but now is bone dry... could it be this water mixed with whatever crap is on my shoes that has misted it?

or is it in fact the heater core and where is that located so i can check?

thanks

[Tyre Smoke]

If it runs hot and aquires an airlock in the cooling system you get tell tale flickering of the temp guage and steam can vent from the expansion tank (just in front of the passenger side of the windscreen) Is this inside or out?

[methodwares]

doesn't run hot (overly hot)... rarely approaches 100 on the temp gauge, usually about 90 - 95 when in traffic and 80 - 85 when in open air... never seen any steam from the car