Where to run the wire.. Usually you can find an existing hole in the firewall to use, if not drill, gromet and attach... I forget how I... Oh, I did not,, They did, I just added some wire under the hood.
Good device by the way, I use one myself.. That 3 minute timer they included in the box... Let me tell you the story about that.
The originals they had problems with folks sitting on the button and burning out the coil... So I was at the shop getting some other work done (YES The BLue-OX factory) and I casually mentioned putting a time out timer in the line....
Well then my Tru-Center failed (Different type of failure) and they sent me a new one...... Complete with the timer.
In one of my other hobbies, 3 minute time out timers are very common.