Don’t argue with Grandpa.
I like @VOLVO_V70 s idea, but how about you needing to borrow the car because yours is in the shop. It sure sounds like the heater core is leaking.
Do as Valvo says and maybe you, your siblings and your parents can pay the bill to fix it.
Split the cost!!!
Then you will be confident that Grandpa will not breakdown somewhere and walk 3 miles to get home in a snow storm.
Yosemite