There are three ways (that I can think of) for the interior lights to remain on in a car with stock wiring:-
1) The Illuminated Entry Timer or Keyless Entry Module. Unplug the module to check if it's keeping the interior lights on. Your car will have one of these modules but not both, or possibly neither. They are located in the trunk on the left side behind the carpet. The Illuminated Entry Timer is directly below the fuel cutoff switch while the Keyless Entry Module is mounted on the rear side of the black vertical module panel.
2) The headlight switch. To turn off the interior lights, rotate the knob clockwise all the way until it clicks. The switch may be bad, in which case you will need to unplug it to be sure that it's not keeping the interior lights on.
3) The door switches located on the door frames between the hinges. If (1) or (2) don't fix the problem, then you'll have to disconnect the switches to make sure that the contacts are not closed.
As I mentioned earlier, this applies to a car that hasn't had any modifications to the wiring.