you can still install it. Either yourself, or a shop.
The shop just can't install the remote start system. Everything else, they can.
Also, this has been talked about, both on here &
http://www.the12volt.com.
You can check there if you want to tackle installing this yourself, very handy source.
Also, since it's a manual, I'm going to go ahead and say "please don't install the remote start feature" - I know ppl say, I will only have it in neutral when i start it and never in gear... there's a reason it's illegal/not recommended...
But if you must, you can also usually just talk to the guys who work there, and they'll install it for you on the side for some cash. Or you can talk to the store owner and say you will sign a waiver assuming all responsibility and that it will be an "offroad" vehicle.
Also, you got ripped off. It's not $600 for a 791xv. Even retail shops sell it for like 300, 400 installed. Ebay when it's up only sells it for 200-300.