... As far as the smell coming from the a/c system, if it is a mildew smell, then it is from mildew building up in the a/c evaporator case. That will occur if moisture stays in the evaporator case too long before draining out through the drain tube that comes out under the right side of the vehicle. There are several ways to get rid of it. You can have a technician remove the evaporator and clean it thoroughly, but that costs money. A cheaper way that will, in many cases, get rid of the odor is the following: 1) Park the vehicle outside with all of the windows open. 2) Run the engine with the a/c on and on the fresh air setting (setting where outside air comes into the vehicle). 3) Turn the temperature control all the way to hot. With the a/c on, it will force air through the a/c evaporator but with it being hot air, it will help to dry out the inside of the a/c evaporator. When you do this, you can also spray a small amount of Lysol into the air intake just in front of the windshield. That will also help to kill the mildew inside the evaporator case. Normally, you would need to run the engine and a/c for about 2 hours to reduce the smell. If it is a sweet type smell, then you may have a heater core leaking and if that were to be the case, the only repair would be to replace the heater core, which, depending on the year and model of your truck can be a little tough and you may want to have a professional technican replace it for you. Source.