Hi, I have the same year nav, I also went the Y25 piping to bypass the prefilter, you have 2 spots under the bonnet where you can fit a second battery on the 03 model, 1st is up front passenger side where basically your pre cleaner and piping would go, directly opposite starter battery but only once you fit the Y25 piping and get rid of the precleaner which is what I prefer and done with mine but havn put battery in as yet, I did get a fair amount actually alot of induction noise when I fitted up the Nissan snorkel and Y25 piping but kinda got used to it now.
And 2nd you can get a battery tray that goes passenger side where your fuel filter is and requires moving your filter but it mounts on the battery tray, you can buy those tray's already to go although I think there pricey for what they are but I'm sure others have done it and found it worked ok for them.
So your call where you want it, me I would prefer to mount it on the first option and is my intention to do so, there is a fair bit of room and could get a decent size second battery in there.
