Unless the tech was trying to troubleshoot a suspect steering wheel switch, there's absolutely no reason to pull the steering wheel to remove an infotainment unit, and even less of a reason to re-install the steering wheel wrong.
Service Managers were created to address workmanship problems like this. I suggest you drive the car back and let him check it out. Mention the new horn/safety issue and I predict you'll have it fixed fast. --And maybe a free, goodwill oil change!
Did the dealer include a customer satisfaction card to fill out with your invoice? Award-winning dealerships do. Service managers get all kinds of awards (like Hawaiian vacations) for superior service by competing with area dealerships, and it's based on the ratings of the feedback cards customers submit.
The Service Manual treats the head unit as a sealed component. If bad, replace. You would need a Denso Repair Manual to dig in and look at it.
As mentioned, removing the head unit is so easy, a cave man can do it. (No offence to any Neaderthals who might be following along. . . )
I poked around online today for new OEM head units and they're very pricey-- one quote was >$3300, which is about as expensive as getting a liver transplant in China nowadays. With a core charge, $1600 may be about right. Still, you can purchase a really nice, full-featured, head unit with a better nav system including installation for less. Be aware you may lose some steering wheel switch functionality and the existing Bose components might have impedance matching issues.
And if this post wasn't long-winded enough, there's another option:
For $700, you can also buy a used head unit off eBay.
Details here: http://www.ebay.com/sch/i.html?_nkw=....c0.m270.l1313
Since it's from a 2007 model, it won't have DVD watching capabilites while driving like the earlier models. Also, be aware that when a different GM head unit is installed, it will require a dealer's Tech 2 to program so it will work with the built-in Theftlock feature.
Hope this helps a little. Good luck!