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.