Central locking system

[Red]

"I just hope the central locking control box doesn't mind getting the 12V input when the door is closed... "
If you are still working on this? That problem is prevented by using a "blocking diode" in the line to the door switch.

Door switch--------------diode---------------12V supply
|
|
to alarm system


The diode has polarity and must be installed one way to work, you can pick some up from any auto electric or alarm shop and ask them to show you which way it is installed for this purpose. In the US they range from a dime to a dollar each, some variation in capacity and price markups.

Blocking diodes are comonly used in just about every sensing line, to the dome lights, brake lights, directional signals or flashers, etc. The more expensive alarms have then built into the main box, the cheaper ones don't. Because you've got an alarm system not just an unlocking system, there are more complicated options in your wiring.

I'd guess the brake light connection is only there so that someone stepping on the brakes disables the remote starter timing, that way the system knows a human is in the car.