In E-cigs it's monopropylene glycol and glycerine. The crap is probably not difficult to remove with more concentrated solutions of alcohol as both are soluble in it. Otherwise, start with lemon juice and white vinegar, acidic plus surfactant then clean with some unflavoured alcohol of some sort!I'm a keen "detailer" IPA as said should work, I'd dilute it. Use a proper glass cloth, you can get a decent one from a supermarket, Minky is the brand. TBH any decent glass cleaner should remove it. Does anyone smoke in the car? I only ask that as fag smoke & especially e-cig smoke can form a layer of crap on the glass, the bit of glass is protected by the sucker & might be a reason as why they show up with condensation.
In E-cigs it's monopropylene glycol and glycerine. The crap is probably not difficult to remove with more concentrated solutions of alcohol as both are soluble in it. Otherwise, start with lemon juice and white vinegar, acidic plus surfactant then clean with some unflavoured alcohol of some sort!
That being said, glass cleaners are pretty cheap.