Gather relevant information about your vehicle on color coding of wires and wiring diagrams of both the vehicle and the alarm system. This information is usually available free of charge on the Internet, by the car maker's or alarm system's website. Also get the vehicle's diagram and study all the documents side-by-side to help you decide what vehicle parts the alarm system needs to monitor.
Identify from the wiring diagrams what colored wires go where and twist them together, then wrap them with electrical tape in groups with respect to the plugs they will connect to. Do this for both the alarm system and the vehicle. Make sure all the vehicle parts you chose to connect the alarm system to are in order and decide where you will mount the alarm system.
Open the panels covering the wires for both the vehicle and the alarm system. Use a screwdriver when removing screws and slide shutters, if any, to fully open the panels. Mount the alarm module onto a wiring harness or support brace in a hidden location. Screw the module firmly into place, or strap it down, depending on the design of your specific system.
Splice all the wires involved in the connection onto their respective slots as instructed from the installation manual of your alarm system. Note that the wire color coding for each vehicle is unique and it is important that you observe where each colored wire connects and its purpose from the vehicle wiring diagram. Solder the wires firmly to their slots using a solder gun and solder wire.
Mount the siren of the alarm inside the engine bay. Drill a hole through the firewall, then pass the alarm's wires through the holes all the way to the wiring panel. The two siren wires are labeled as positive (+) and negative (-). Connect the positive wire to the alarm module's positive (+) siren output and the other wire to the ground. Replace all the panel covers and test your alarm system.