(((((((((( janniebug ))))))))))))
I know this is so very hard, emotionally, it hurts to see someone so ill, especially a family member, I've been in a similar place. Try to keep the faith, I also can relate to that awkwardness of not knowing what to say to a person, who is so very sick and is fighting a terminal illness, I wish I could offer some ideas,etc. but I'm lost on that.
Now, your car situation, is it only the heater? Is the car running fine otherwise? If so, don't worry right now about next winter, you'll have time and maybe come up with what you'll need to make the repair. The main issue at the present that you mention is your dad and the support you are trying to give or be there for.
I had a car that the heater was shot, but it ran, and months after when we were able to have it fixed, we did, funny it was summer time when we got around to it, sooooooo during the cold season, I was bundled up, bulky when trying to drive, with gloves and all the other winter clothes, but I got through it. I don't think it would have been possible if the defrosters weren't helping. The heater must have only been partially shot, but it never was warm in the car for the driver or passenger(s).
Try to take care of yourself, physically and emotionally, let the car be secondary as long as it is running.
I'm sending you and your family special thoughts and wishes for strength,healing and innerpeace.
Please take care,
DE