Everytime I have my car washed or it's raining my windows get steamy when I use the heating. I need to use the air con to get rid of the steam. Does anyone know what's causing the steam and what is the solution to this problem.
If it is raining, then yes you should need the heater and hence the aircon on to stop misting. This seems normal to me even without any leaks. I can remember cars without heaters, we had to open windows and keep wiping the screen when it rained - ridiculous.
I have no idea which heater/aircon you have but mine runs up the aircon to dehumidify when in full heat defrost mode. It is supposed to be on.
Turning the aircon off can result in the pump not running for long periods and then it loses pressure. I found out the hard way over the winter when I had a wire-off at the compressor.
I think whitenemesis nailed it, either a leak or a damp pollen filter.
Having said that, I recently had a similar problem on my w124 estate only to find a 1.5l bottle of water had spilt in the boot liner and remained hidden by the load cover. 1 old towel later, and problem solved.