c5 2.0 hdi Strange Immobiliser / Battery Drain Fault

[grendal10]

im having a few problems with my c5 2.0 hdi 2002, first problem was the battery was draining completely if not used for a day to 2 days, which turned out to be the rear interior light, if its turned off the bulb still has a small glow . i then found out if the bulb is removed or the interior light is unplugged the car wont start and comes up with immobiliser fault, if the bulb is plugged back in and ignition turned off and back on the fault goes and the car starts fine. just wondered if anybody had any ideas?

[white exec]

That sounds like a failure of supply to some part of the car concerened with starting. The interior light is temporarily providing a supply, instead of the IGN switch - which is why the battery goes flat: akin to leaving the IGN switched on. Can't help further without the circuit diagrams.

[GiveMeABreak]

Have you 'added' anything to the car such as any electrical items or changed the Stereo at all - or basically spliced / cut / added / removed any wiring?