When I turn on my defrost it smells like exhaust

Welcome to 2CarPros. The first thing would be to track down if the smell is gasoline or exhaust. A gas smell could be from a leaking o-ring on an injector or fitting under the hood or it might be vapor from the EVAP vent valve.
Does it seem to smell more when it is hot outside? Any other times it smells?

Does it seem to be stronger after you fill up? Do you smell it outside the truck as you walk up to it? How about if you open the hood, smell it more then?

Hello,

This sounds like a classic fuel injector leak which can be tough to spot. With the engine running use a flashlight and check each injector for wet fuel. When you find it can you please shoot a quick video with your phone so we can see what's going on? That would be great. You can upload it here with your response.

Did you get a chance to see if you could smell anything with the hood open? If so it would tie in with Kens post above.

Did you check the injectors while the engine was running? Also look for a stain of past leaks as well.

Okay, try this: Turn on the key don't start the engine, turn the defrost on, do you smell it now? Now start the engine, any smell? Cycle through heat, AC, defrost. Anything?

Okay, if you smell it when the engine is off but defrost on, it is possible it is mold/mildew growing in the HVAC unit. Somewhat a common issue with modern vehicles. First thing to check would be the cabin air filter. It is in the dash, hiding behind the lower glove box. To get to it you flip up the upper door, then flip the lower one down, remove the 4 screws that hold it in place and gently pull it out, there are two clips at each side so it may come out a bit hard. Behind it will be a door that folds down and inside that is the filter. It is an ACDelco CF188, Hastings AFC1626, Wix WP10129, NAPA FIL 230129, and a few others depending on where you look. I have seen them come out disgusting if people don't know they are there. While you are getting that pick up a can of vent deodorizer. The stuff I use is sold at the local dealer and is a spray foam, you basically shoot it into any opening for the heater you can find including the outside air inlet, it foams up and kills off anything living in the duct-work.