HVAC recirc/external air valve location?

[newbroughr]

So, I recently found out about how since 2003, GM decided to nix cabin air filters in all of their trucks. I suffer from bad seasonal allergies, and I work at a very dusty location. As such, I want to try to solve this problem.

 

After searching around I was able to find DIY retrofit instructions, however after I dropped the plastic cover in my truck, I saw that my HVAC didn't quite look like the HVACs in the pictures/guides I'd found. The flat spot where the slot would be cut is not flat in my truck, but tapered. It also lacks the spot that a screw could easily be driven into. Because my set up is not quite the same as the pics I've seen, I don't feel confident cutting into anything.

 

Aggrivating me even more, I've noticed that whenever I turn the fan off or back on, the heater/ac controls automatically switch from cabin air recirc back to external air. I bought the truck used at 83k, and I couldn't help but notice that the button for cabin air recirc is very worn, suggesting that the previous owner was hitting it all the time, just like I am.

 

I've seen the question asked a hundred times all over the internet on various GM truck forums about whether filters can be put in. The only thorough guide I found was from TheBac on dieselplace, and a random one I found on an avalanche forum who did basically the same thing as TheBac's retrofit. So, I'm gonna guess no one will have anything to add onto what I found, but I'll put it out there that if there is anyone who thinks they might be able to help, please chime in.

 

Alternatively, what I think I might like to do is to disable the valve that switches between external air and cabin air recirc. If there's anyone who knows where that specific part is located, and how I might be able to disable it or lock it in place, please let me know. I have a crew cab GMT-900 2007 Sierra SLE.

[MikeNH]

I thought most of the retrofits were on the 03+ 800-series trucks, not the 900 series? If you have automatic climate control, this is what you'll want to unplug I'm pretty sure, though be aware that using recirculate on humid days can fog up the glass.