Smell antifreeze and windows fog up
Heater core is leaking
Yep that is what I think too.
I am sorry to say but it is the heater core.
Check under the dash to see of there is any wetness. It may be the hose that connects to the heater core or even to the firewall. The hoses are cheap to replace but can be a pain. A lot cheaper and easier than replacing the core if it is not leaking. If it is the core, replace the hoses also.
The hose is under the hood not under the dash fill of floor board and see if the carpet wet.
Some vehicles have the heater core inside the cab of the vehicle, so it has to be connected from under the hood somehow. What better way than by a hose, aka heater hose. Yes there is a hose under the hood running to the firewall. Some continue through the firewall to the heater core, others connect with a metal fixture that goes through the firewall and then a short hose to connect to the heater box. Best to check every aspect of the possible source before tearing into the heater box removal. :-) Just suggesting. I may be cheap by at least I'm lazy.
The heater core may be under the dash but most of them are made so that the connection are made under the hood if you would like I can draw you a picture.
Looking for a Used Silverado 1500 in your area?
CarGurus has 167,240 nationwide Silverado 1500 listings starting at $1,800.
Search Chevrolet Silverado 1500 Questions
Chevrolet Silverado 1500 Experts