1990 Volkswagen Golf Smell when heater is running

I had that problem two years ago. One day out of the blue I got a foul smell when I turned on my heat. I took it to the mechanic and he said it was my heater core - that antifreeze was leaking and that this is toxic. You have to fix it because otherwise you won't be able to use your heat. This is an expensive job - it costs around $1000 to fix your heater core.