I was curious to see if I could combine a flashing LED circuit with the LEGO TARDIS (#21304 - http://shop.lego.com/en-US/Doctor-Who-21304). With a little bit of drilling, I was able to mount an LED under the TARDIS roof and show through the clear studs. The flashing circuit (breadboard and battery) is still external to the TARDIS.
Step 1: Remove the "light" From the TARDIS Roof
Remove the "light" portion from the TARDIS roof.
Step 2: Make Small Drill Hole
Drill completely through the roof section using the 7/64 drill bit. Note: I used some handy "round 1x1 bricks with open stud" as a drill guide.
Step 3: Drill Larger Hole to Hold LED
Remove and set aside the portion of the roof that sits above the six-by-six blue plate.
Flip the six-by-six blue plate over.
Drill a larger hole through the two pieces (blue plate and gray plate) with the larger 3/16 drill bit.
Then re-assemble the roof.
Step 4: Flashing LED Circuit
Author Note: I had an existing Flashing LED circuit using 555 timer chip from another project. The LED is a super bright white. I will need to develop/link to separate instructions for that.
Step 5: Insert LED Into TARDIS Roof
The 3/16 hole we drilled is just snug enough to hold the LED in place.
Step 6: Re-assemble TARDIS
Place the TARDIS roof with LED back on the model. Run the wires out through the "doors" and carefully close the "doors". Fortunately for us, the "doors" are at the back of the model!
Step 7: Done!
Attach the 9V battery.