I've installed an extra 12V source in my car and everything is working.
While installing it I removed the positive terminal from the battery and left the negative connected. (Resting the positive on the plastic cover on top of the terminal) Once the work was done I just reconnected the positive side of the battery.
I've been reading up saying there could have been issues if I had accidently touched the spanner to a metal part of the car and created a short circuit. I didn't have any issues (Didn't touch any metal with the spanner) and the only thing that happened was the terminal sparked a bit while I was pushing it down while re-connecting everything.
The car is all working but I just wanted to make sure it wouldn't have any adverse effects on the electronics in the car as I've read I was meant to remove the negative terminal first, then positive; reconnecting the positive first.
(which i obviously didn't do just disconnecting the positive while doing work)