If you could prove that it was installed incorrectly to the shop'* owner or manager, the best you might get is a refund for the installation. Unfortunately, I still don't think the shop has any legal liability if the car is broken into or stolen. It could certainly hurt their reputation if word got out (how about a Letter to the Editor in your local newspaper), but it doesn't seem like suing is a viable option. Call the owner/manager back and politely inform him of these options, and see what they might be willing to do for you.
Current GMs: '95 Chevy Suburban C2500, '69 Pontiac Executive
Former GMs: '83 Buick Park Ave, '73 Buick Electra, '92 Chevy Caprice 9C1, '98 Pontiac Bonneville SLE, '99 Olds Aurora, '00 Buick LeSabre