Have not heard of a heater core failing for well over 10 years. I would be surprised if it is but hope it isn't, as it is a real PITA to replace in older cars, the Golf will be a nightmare.
LOL, and there wasn't all of the sound deadening and under dash trim then. Early 60's Mustangs and Falcons were fun, you needed to remove the entire housing and open it up.
I knew a mechanic at a Chevy dealer who traded in his old 'Vette because he didn't want to replace his heater core. Guess who the dealer had do it.,