where is the filter located under the hood?

On the passanger near the firewall you will see it its marked as cabin filter.

lol OH that ought to be easy to find

Here'* another useful link with links to lots of pictures:

http://www.bonnevilleclub.com/forum/...t=cabin+filter

My 2005 didn't have one. I installed a charcoal filter with frame. More recently, I replaced that filter with a non-charcoal refill (no frame)

Don't know if this means much to anyone, but my GXP came with a cabin filter already installed. I came across it by accident when messing around just getting familiar with the car, and lifted that latch open to find a filter sitting there.

I think its kinda cool to have the air coming into the car filtered.

Thanks for the info. Yesterday I installed the filter received from Rockauto. It was pretty easy.

The previous go-to link is dead, so here'* one that works:
https://www.gmforum.com/t246667/

It'* hard to believe that was 2 years ago. The filter at PartsAmerica (with charcoal & frame) is up to $54. Ooow! 75% increase in two years. Kinda expensive for an owner optional part.

Same filter w/ frame at RockAuto still at $31. (+$8 ship)

Latest Cabin filter pricing for Fram CF8804...

Available from Amazon at:
Amazon - Fram CF8804 Cabin Filter

... for $20.55 after $3.00 rebate and with free shipping, if you order a total of $25 of qualified stuff.

Rebate: The Rebate