Hey that's a pretty cool way of doing things.
If you ever do decide to add something else, running wire to the battery really isn't that hard and if you do it right, you don't even have to drill any holes in your firewall.
You can buy inline fuse holders (Up to 30A blade type) with some 12AWG (usually) wire on either end, perfect for smaller applications like this. You just connect either end to some power wire, and hook the power wire to your battery with the fuse ~6 inches away (New battery terminals are the best way to do this, but you can also use ring terminals on the wire and use the bolt on the existing battery terminal to secure it in place). The other end goes to your device.
There is a rubber grommet in the firewall where the stock wiring comes through. It has a big rubber plug sealing it in place. I just poked a tiny hole through it with a clothes hanger, and pulled my wiring through that. The bigger wire being forced through the smaller hole seals it up nicely too. (Side note: You never want to run something through the firewall without sealing it)
Anyway, here's a pic of my 4AWG power wire running from my battery, you can see how I used a ring terminal to connect it to the battery post.
i got it to work perfect muahhaahaha.. they sell these "FUSE adapter" thing.. but some weird ass reason they're REALLY hard to find.. 
