Originally Posted by bmsgaffer86
hrmm... The SAFEST** way to do this is through a circuit using a timer and PWM (Pulse Width Modulation).
I thought about that as well, but you run into issues with dimming to full off via PWM -- eventually you're going to reach a pulse width that will be present itself to the eye as an annoying blink unless you use a high enough frequency, and high frequencies demand a load driver that'* capable of quick switching. Solid-state relays are probably out (most of the ones I've worked with aren't capable of turning on and off reliably hundreds or thousands of times per second), and they're expensive anyway, so you're most likely stuck with power FETs. Depending on how much power the interior lights draw, that might get expensive too.
It also might be difficult to get a 555 timer to automagically start dimming itself; most of the PWM circuits I've constructed used a pot to manually vary the pulse width. I'll look at it and see if there'* an easy way of doing it (unless you've got the answer already, in which case I'll shut the hell up