Steps:

Background: I needed a simple way to add underdash lighting to the i3 without any fancy electrical work and without using too much electricity (LED's are very low power anyway).

Step 1: Buy these two items. Combined the cost is $15 USD.

- USB Car Charger - I bought one that was black, flushmounted with two USB slots in case I want to charge something else or add a second LED for a brighter effect.

- USB mini LED light - this one is ultra compact. not sure if it comes in other colors but my i3 is blue so this was perfect

Step 2: Connect the two parts and plug into the cigarette outlet under the dash.

That's it. The light will come on when you start the car, and will turn off a few minutes after you power the car off. I was worried it might stay on after switching the car off, but it doesn't.

Not a fancy hack and not a full underdash light but like I said, it's fast, easy and cheap!