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Why do my VW Golf's rear windows mist up in cold weather and how can I fix it?

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Car Clinic: Foggy windows

Q. The rear windows in our 2008 Volkswagen Golf TDI often mist up while driving in cold weather. They can also take several minutes to clear when first starting up in the morning. Why is this? RJ, Basingstoke, Hampshire

A. It is likely that water is getting into the car. In the warmth of the cabin, it evaporates and then collects as condensation on the cold window glass. Find the leak and your windows will become mist-free once more.

First, look for signs of damp on the boot floor carpet and under it in the spare wheel well. Beneath the wheel there’s usually a drain hole covered by a rubber plug: make sure it’s properly in place to stop water getting in.

Next, pull back the carpeted side panels in the boot. If there’s damp behind one of them, it suggests a leaking rear-lamp cluster. Remove the cluster — it’s held in by two 10mm nuts, which you can access from inside the boot — and check to see if the rubber seal around the edges is in good condition and not bent out of shape.

Another possible source of leaks is the rear screen. Sit in the loading bay while a friend hoses water around its edges and you’ll soon spot any ingress. Use the same procedure to check the tailgate seal.

Leaky seals can often be plugged by judicious use of Granville Clear Silicone Sealant (£4.99 for a 40g tube from halfords.com). A leak in the tailgate, however, might require complete replacement of the seal — but not before you give it a really good clean with a specific vinyl cleaner such as Autoglym Vinyl & Rubber Care (£7.49 for 500ml at halfords.com), which can often soften up the seal enough to restore a good fit.
Sunday Times Driving car clinic expert: Dave Pollard

INSPECTOR GADGET
Dave Pollard has written several Haynes manuals and has tested just about every car-related accessory – read more from Dave here.

GOT A PROBLEM?
Email your question to carclinic@sunday-times.co.uk or write to Car Clinic, Driving, The Sunday Times, 3 Thomas More Square, London E98 1ST, with a daytime phone number, your address and as much detail about your car as possible. We can’t reply in person, so don’t send original documents or SAEs. Advice is given without legal responsibility.