The timer was long ago discontinued by GM. I has only been recently reproduced and runs around $120 (Oct 2008). The timer function can be easily bypassed at the connector. Remove the Timer Module. If you place a jumper wire in the connector at the outermost pin (WHITE wire) and the second pin from the other end (also WHITE wire) the interior lights will simply shut off when the doors are closed.
Interior lights often stay on after the door is closed because of a faulty relay. There could also be a broken wire or a short circuit.
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Head lights
The interior lights fuse on the astra 57 reg is located on the door hatch.
I believe if you turn your gauge panel dimmer switch all the way it turns on the interior lights when all the doors are closed
Usually the problem is due to a bad ground as the car is mostly fiberglass. Also check the bulbs.
Under. Hood
The dimmer switch when turned just past high, turns on the interior lights. There are switches in the door opening that turns the lights off when the door is closed. Either one of these can cause the lights to stay on if they fail.
Found out the problem, it was the door ajar button
Just to the right of your headlight switch is a dimmer wheel for your gauge lighting If the dimmer wheel has been rotated all the way up , your interior courtesy lights will stay on ( even with your doors closed ) If your interior lights are on with a door ajar indicator light on in your gauge cluster , either a door ,your liftgate , or your liftgate glass is not closed or you have a sticking door ajar switch (Helpfull)
The interior light on an 87 cabriolet is located above the mirror just above the windshield.
fuse #17 in the fuse box located under the rear seat bench