I did the same for a few reasons:
USB Chargers are Cheap. They power my phone and USB Lights.
I can buy a cheap 12v Cig Lighter Fan also.
2 x 1/4" Guitar plugs solder on:
http://frys.com/product/1646454
1 x 12v Lighter Splitter:
https://www.amazon.com/Electop-Cigar.../dp/B01HEV8LHG (found mine at Walmart for 7 bucks)
Cut the plug off to give me two female's that I could wire into each 1/4" guitar plug.
Then I can use:
12v Oscillating Fan
USB Phone Charger
USB Dimmable Light
Makes perfect use of the plug as it is, I like the 1/4" design because if you jerk it out or roll on the wire, or are not careful, it pulls right out without issue or breaking.
Just meter test it for 12v+, which I remember was CENTER on 1/4" plug.