Thought i would share a few simple circuits other members my find useful. sorry for diagrams but description is in the pictures
Diodes and relays are used in circuits
diodes are tiny switches that allow power to flow in one direction but block it flowing
back and are very cheap to buy
relays are mechanical switches that are operated by applying 12v to the coil which then switches higher currents across the relay

great for adding accessories, alarm led, better horn, secret alarm etc
if you have any question just message me

the circuits are examples of what is possible, they can be adapted to what ever use you have , like the secret alarm, you can fit a hidden switch that will need to be pressed to avoid the alarm going off, remember they have your keys and think they can take your car, you can make it more complicated requiring several switches to be in certaion positions to disable ( ie: brake pedal pressed while left indicator on and heated rear window on etc or as simple as hidden push button to disable, once secret alarm has been set off can only be turned off with ignition on and secret button pressed ( switch activated ) if you need any specific circuits message me and i will try to do a circuit for you as these are only simple examples
Others available like adding a second battery without the cost of a split charge relay etc

have fun