Both cars wont start - 06 G6 GTP and 07 VW Golf

do you run synthetic oil? that makes it easier to start in the cold
you said they were pluged in so i assume the have block heaters-good
might need new batteries with better cranking amps and baterries CAN freeze and be ruined so a plug in battery blanket is a pretty good investment...........also if you have any water in the fuel that would do it too.....heet works good.....pretty much its just rubbing alchol LOL

just keep in mind dave that once a battery freezes no matter how old or new its basically junk, you can buy a new battery today and if it freezes tonight and wont start in the morning the new battery is junk..........a frozen battery wont ever hold a proper charge after it has frozen......like in the case of the battery on my blazer in wyoming, my first winter in MT, it got -40F ad the battery froze and after that even in warm weather it wouldnt hold a charge longer than about a day......PFYC sells electric battery blankets for the vette for $25 ive got one on order and hoping it will fit my battery as i put in an optima yellowtop for 179 and dont want it freezeing..........most of the time even in MT and ND it doesnt get cold enough for batteries to freeze but when it does get down to -40 i dont want to have to replace the batt again just cuz it froze and turned to junk LOL