Its proberly the bracket that attaches the panel to the airframe. The metal conducts the heat to the outside and chills the patch down to below freezing.
All the pax sweat and breath condenses and forms the frost.
Up the front on sometypes you have to watch putting your arm/hand anywhere near the window fittings on the unheated ones. You only do it once but it is quite possible to get a cold burn or even stick to DV window rails. And once you go through the freezing level you have cold dripping water down your arm for much of the approach.