1999 GMC Jimmy Can smell anti freeze in cab very strongly

Sorry, It's your heater core. They are not real fun to replace. T stop the problem until you replace the core you can use a steel tube and bypass the core under the hood.

Just helped my brother get his replaced. what a pain. I'd reccomend sending it to a shop or finding the neighborhood car guy to fix it. too me 6 hours of work, mostly trying to find all the mounting bolts for the dash board.