The cell phone camera makes the light far less diffuse that it truly looks in person... Hopefully Video of the color changes should be up later.

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To Begin:
Go to Guitar Center and buy the cheapest bass you can. In my case this happened to be a leftie Ibanez GSR something or other (even though I'm right-handed. It was $88. Great start...

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Decide on the area you want to replace with the Plexi to be illuminated.  I'd say about 1 1/2" from the edge of the top. If your guitar has the same slope on the bottom, I'd recommend staying away from that.

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Take the router and be Effing careful. I just eye-balled my design into the bass. It turned out really well, but could be better if you took the time to make a jig.

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Create the space for the wires. I used a large keyhole type router bit.

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Assuming your circuit is all set up go ahead and lay in the LEDs and wiring. Hot glue works well for holding everything in place.

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Cut the shape for the infill from Plexi. Accuracy is key. Once the shape is right  - frost it and make another - frost that one to make the light more diffuse (and to cut down on fingerprints).

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I replaced most of the original guitar parts  - Bartolini pickups and a 250k tone pot went in.

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Then the LED circuit and batteries and Guitar electronics went back in.  Pictured Below  - Tone, Volume, On/Off toggle, program up and down buttons.

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The LED control is just a few programs and uses Pulse-width-modulation to control fading and color selections. You can use whatever you want to control these. This isnt really meant to be a tutorial, just a prototype and how it worked out.

I'm working on a custom body with more, lower power consumption LEDs integrated a little better to illuminate the face more uniformly. When that is near completion. I'll include a more in depth tutorial (with pics from a camera, not a cell phone) on both the circuiting/programming and construction.

This version will be for sale, I'll also build them for you if you want. Turn-around time is 2 weeks and price would depend on the feature set/bass/bass electronics you'd want.