I have a 2007 tahoe that I just bought used and everytime I turn on the A/C it smells kinda moldy our something like a wet dog. After being on for a while it either goes away or I get used to it. Since there is no cabin air filter to change what else can I do? And where does the air come from that feeds the a/c? Thanks in advance for the help.

 

Welcome to the forum.

The evaporator drain may be clogged and the retained condensate causes mildew before it can evaporate. Once the clogged drain is remedied, there is a product that can be spray into the system that will take care of the smell. There is also a rear evaporator.

 

Where would those drains be? I do know that water leaks under the passenger floorboard(outside the vehicle) when the a/c is on so that one in the front should be good but what about the back. Any idea where the air comes into the a/c system?

 

back one will drain right behind the back passenger tire - you can look up and see the small drain
I think it pulls outside air in from right at the base of the front windshield just like nearly every other car.
but unlike most modern cars there is not a cabin filter (as you found out and not good IMO). a good tip is to run the fan for a minute or so before you turn off the truck, with the AC off. what we do is when pulling into our neighborhood I will leave the fan on but turn the AC off, this will help dry off the coil