Updated April 15, 2009
I have that Pioneer unit in my 05 SUT. It works great! I had them hook it up so that all of my stock steering wheel controls now control the Pioneer unit. I use the source button on the steering wheel to scroll between am/fm radio, cd (I mounted a 6 disc changer under the rear seat), sirius satellite radio, dvd, or the local tv tuner. It also has a small detachable remote that mounts on the steering wheel that controls everything as well. You can control the unit in these two ways or use the touch screen directly on the monitor. It of course has navigation and I also mounted a rear view camera that comes on automatically in reverse or I can select to have it on while watching a dvd or using the navigation. The On star works the same because there are different speakers than for voice activation for the Pioneer unit. You will be extremely happy. I intentionally got the stock stereo from the factory w/my SUT because the $1700 option for the factory navi wasn't worth it too me when you can buy the Pioneer unit with all those features for a little over $2000.
I had the Pioneer Nav (2005 version ACIS) in my Denali. It is suppose to be one of the better aftermarket Nav systems.
Here's what you lose!
Speed sensing volume. (No way to get this back)
Rear Radio (No way to get this back)
Steering wheel buttons (Need Module)
XM Radio Need Module)
On-Star (Need Module)
There are modules that you can add.
You can add the XM adaptor (Your stock XM will not work with the Pioneer system)
You can also add the Steering wheel button adaptor (Both are pricey!)
If you add the steering wheel button adaptor, you will have random use of your steering wheel buttons. Which means they will work most of the time, but they are intermittent. I found that occasionally the volume would change the station or the buttons wouldn't work at all. Also you will not be able to adjust the volume of the On-star. This sucks big time.
The Pioneer system is voice activated, and it gets confused very easily. Gas Station my come out as Hospital.
And you cannot program the Navigation if your moving (You might be able to over ride it if you're a signal expert) they have linked the use of the nav system to the GPS antenna so if it senses motion you cannot program the Navigation system. You have to stop. And if you're trying to locate a restaurant in an area you're not familiar with. You will get every crappy little restaurant first, no matter where you are. And most of the well-known restaurants won't even show up.
I wasn't happy with the Pioneer I wouldn't do it again.
I have a 150psi ViAir compressor and a 3 gallon tank mounted in the jack compartment on my SUT. I couldn't really find a good place to mount the tank under the truck. You may have better luck with a slim tank.
If you're going to use the system to power an air horn keep in mind that air volume is key, not so much the pressure. Larger hose, larger ports on the tank, etc make a big difference.
I mounted these horns under the hood: