I just installed a set of tweeters there back in August. I do not have the Bose setup either. I used two 3" tweeters that I purchased on ebay for something like $19.
The panel up on the dash just lifts up, there are no screws holding it down. After removing the panel and the Ambient Light Sensor, I cut the cloth covers away from the speaker mounts, then used a piece of wire from a coat hangar to route and pull the speaker wire through to the the back of the HU. Routing the wire wasn't hard at all. Took me no more that 20 seconds per side.
Once the speaker wires were routed, I connected them and used self tapping screws to hold the speakers down.
I then tapped into the two wires (for the front speakers) with a few of these:
http://www.midterminc.com/media/5602.gif
If I'm not mistaken, the front
right speaker wire colors that I tapped into are green and brown. The
left wires are grey (with and without the black stripe).
After installing, the sound inside of my Envoy is much more "full" now. I'm very happy with it!