Will '06 stereo head unit fit in a '99 Will it sound bad compared to my Kenwood

Here are some examples, I don't remember needing an adapter

Center dash console cover

center dash bezel

Oh, ok. So, what you're calling the console cover, I usually call the bezel. But I think your terminology makes more sense.

I picked up a 2003-2006 stereo recently and indeed the connection/mounting points are different than on my 97–02. Will try to post some pics sometime.
 
This site contains affiliate links for which Jeep Wrangler TJ Forum may be compensated.