Hi
Tucked behind the passenger ( Left hand ) side headlight are 3 large wiring loom connectors, 1 x Grey, 1 x Brown & 1 x Black.
These are the cause of all sorts of electrical faults on Peugeots / Citroens because nearly every circuit on the car passes through these including the anti theft device.
Have you checked the terminals are clean & making good connections inside these plugs?
Notice the classic piece of rubbish design, 1 male terminal pushed up agaisnt another male terminal, They just twist together, Theres a locking collar on the plug, Just twist it to undo & do it up again.
The terminals get dirty & then the fun starts with the electrical faults.
Give the terminals a clean with WD40 or similar & a tiny bit of adjustment with a screwdriver if needed so they make good contact with each other.
Make sure you have the radio code, shut the alarm system down ( Its in the handbook ) & then disconnect the battery negative terminal first before undoing any of the 3 connectors.