lights off after 5 min

there are several ways one might go about this - as i'm sure you're probably aware, many vehicles come from the factory with just what you're talking about (the crown victoria comes to mind).  anyhow, it shouldn't be too difficult to do - basically you need to install a countdown timer, powered by your dome light, which will disconnect power to the dome light circuit when it reaches zero, and is reset when the ignition is turned on.  i know there are several IC's on the market that can be used for counting down - furthermore, there are some that are specifically built to act as a programmable countdown timer that have reset functions - i'll have to look up some part numbers to be more specific - this shouldn't be too difficult though so long as you've got at least marginal soldering skills, etc - i'll do some looking and leave another post as soon as possible.  there may be a module you can buy too that will perform this function, i wouldn't doubt that it exists... anyhow, i'll see what i can dig up