had a similar problem with a BX once. It turned out that the switch
for the boot light had shifted a little so it did not detect that the
boot was closed. So the light was on constantly.
An easy way to check for this is to fold down the seat so you can see
the light when the boot is closed.

Probably no use at all, but used to have a Montego.  Every time it was
parked in the sun on a hot day the battery  went flat.  I finally tracked it
down to the interior light in the boot.  When it was really hot the wires
must have expanded at the point where they bend when the boot is opened and
shorted.  Although the light came on under these conditions, it was too
bright to see (assuming you could see inside the boot in the first place)

Peter