Had exact same problem on my old marea, Its an eay fix - just take the
bulb out of the boot light.

On these mk 2 mareas and brava/os post 2000 ish, theres a safety feature
on the central locking preventing the doors from locking if any of them
are open. Detection is through the interior light circuits.

Any load on the courtesy light cuircuits and the central locking will
pop open all your locks within a second of you turning the key to lock
them.

There are TWO seperate courtesy light circuits, one for the 4 passenger
doors and one for the boot light. If your interior courtesy lights are
working properly the fault must be with the boot light.

It's a simple enough task to trace the wiring from the microswitch on
the boot lock stricker to the boot lamp and check for damage. Although
the bulb holder is a poorly designed thing without any shrouding around
the contacts and is probably the culprit. If the live feed shorts the
fuse blows, no boot lamp no problem but if the return wire shorts it
will place a load on the circuit and confuse the central locking to
thinking the boot is open.

It's even more confusing because the boot lamp itself goes out
automaticaly after 10 minutes or so so there's way you can just look
through the rear seats and check if the light is on. don't ask me ask
FIAT.

If a problem can't be found and rectified, the simple answer is to
remove the bulb from the boot lamp. Without a bulb there won't be a load
and without a load the doors will lock. Simple.