Anyone who has attempted to install internal lighting in plastic structures has faced two headaches: the incandescent lamps normally used are bright enough that the structure's walls tend to "glow," and the light also throws into relief the fact that there is nothing inside the building (obviously the latter point is moot for those very few of us who super-detail their structures by placing items inside).
The usual solution for the former is to paint the inside of the structure black. This presents new problems, especially if you're dealing with an already built-up structure, because the edges of windows and doors need to be unpainted so they can be glued to the inside of the walls. However, leaving them unpainted will cause "glow" light around the edges. It's a Catch-22.
My solution is to replace the incandescent light with yellow LEDs right behind each window or door which is to be lit. This looks great in the dark, and the LEDs are dim enough that they won't make the walls glow.
First, determine which windows and doors will always be dark, and attach black construction paper behind the glazing and any other details such as curtains. Easy.
Second, for windows/doors where you want nighttime light to appear, attach regular white paper behind the glazing. Now, using regular 5mm yellow LEDs (not the super-bright ones), place one LED behind the white paper on each window or door, sitting 3mm or so back from the paper. Wire all of them in parallel, making sure the anodes and cathodes are all going the same way (i.e. they should all be wired with the same polarity). To this parallel LED cluster, attach one 1K-ohm resistor in series (it doesn't matter whether this is attached to the anode or cathode side). You can try reducing the resistor's value if the LEDs appear too dim, but don't go below 680 ohms.
The last step is to put a full-wave bridge rectifier between your accessory power (I'm assuming you'll be using 16-20V accessory power to light the structures) and the LEDs with resistor. Make sure you match the + output of the rectifier to the anode side, and the - to the cathode side, with your resistor in series on one side. If you omit this step, the LEDs will only be using half the power cycle since they only pass current in one direction, and they will flicker and appear dim. Since each structure will only be drawing around 15ma or so, one 1-amp rectifier should be good for quite a few structures.