Remember seeing a thread recently re. lights on reminder. Simplest way is to buy a Vauxhall LOR unit at about £14 new or £3/5 S/H The unit looks like a standard car relay with 3 .25" push on terminals. A new one emits a nice electronic sound whereas an old on just buzzes. Can be found in most Cavaliers/Vectra/Corsas and Astras Simply wire as follows:

Terminal 30 - car light switch (ie + 12v when lights on)
Terminal 15 - ignition (ie 12v + only when ignition on)
Terminal 31b - door courtesy light switch

I picked up the light switch + from the fog light switch, the door courtesy switch at the interior light and the ignition + from the cigar lighter ie all close together below the instruments. (9/01)