Would it be possible to modify the electric circuit of the interior lights (the ones on the roof) so that they fade?
It would be nice to have lights that turn on fading when opening the car, and that fade away after you leave it: I have such a thing on the Lancia Lybra and was thinking about upgrading the Tempra...

You can get a small box from a scrappy out of a car that has it, my mate got one out a carlton for his heap of ****e nova.

bravo does it but my marea didnt i was going to mod it to do so, so yes you could.

I thought the dimmer was part of something much bigger and so I was thinking about self-building a circuit, if simple enough. Have these boxes almost standard connections or there are some which are better suited for a "conversion" application?

you can use the dimmer relay from any car that has one and wire it up to your car. very easy to do. you simply wire it into the light's circuit and whatever you want to trigger the on/off delay (such as your central locking) almose every car's dimmer relay is the same. you can buy dimmer relays on ebay for cheap.