The Gal that has the faulty Heater Core in her '90 Jetta should check with her local Volkswagen dealer to see if there is a heater core recall for that model year. There was a recall issued back in the '90’s that covered the heater core in my '86 Jetta and a few other years–my '86 is the same generation Jetta as the 1990. I never had any trouble with my core but about 10 years after the recall was issued I asked my dealer if he would still honor the recall, and he said yes! Got a new heater core ‘for free’ on a car that was at that time almost 20 years old! Worth a shot for a ‘free’ repair!
The show is in reruns so who knows when the call was made? The brothers have retired.