I have heard good things about these have and used to know some of the guys. Been a while, so I dont know if they are still there:

Western Radio
278 University Boulevard, Denver, CO 80206
(303) 320-4913

Same goes for these guys:
Sbs Electronics
6201 West 44th Avenue, Wheat Ridge, CO 80033
(303) 424-7032

And a buddy got his with remote start/security system (CompuSTAR) put in in Longmont.
Perhaps here:
Accent Auto Sound & Accessories
633 Kimbark St
Longmont, CO
Phone: (303) 772-3222

I would go to any of those and that Compustar is ROCKIN! One of the best alarm packages I have ever seen.
www.compustar.com