check your vin on one of the online vin decoders, alarm prewiring shows up as a feature on the report if you have it. I put my alarm in (9/2008 build E92, so it was prewired) myself and got it from Tischer, they were (maybe still are?) running a special where the alarm was about $250 and free shipping, but had to have the dealer code it in for me for another $125 or so. I installed the Sirius module at the same time so I would only have to pay one coding charge.
Edit: I feel obligated to mention that the main reason I installed the alarm was to get the windows-roll-up-with-keyfob functionality but that didn't work. Bummer. I left the chirps enabled though; they're obnoxious as hell but it is nice to have confirmation that my doors are locked. The lights blinking I haven't figured out yet under what conditions they do and don't work, but it doesn't seem to be a reliable indicator that the doors are locked.