As of 2016, AutoSecurity.Tripod.com, CCP.edu and SelfSolved.com provide car alarm installation diagrams or links to downloadable diagrams. These sites show images of installation diagrams for specific systems.
SelfSolved.com shows a car alarm wiring diagram for the Performance Teknique Car Alarm ICBM - 7071 on a 2002 Ford Crown Victoria. AutoSecurity.Tripod.com has instructions and a diagram for installing the ATEK 4000-4800 Remote Start/Alarm/Keyless Entry Combination System. CCP.edu presents information and diagrams for the factory alarm on an MKIII Golf or Jetta. The site displays a generalized alarm wiring schematic for all Golfs and Jettas.
Every car alarm system and most vehicles' factory alarms are unique and have unique installation and wiring diagrams.