Has anyone, this seems to make sense, put an extra battery in their van.  I
am parking my Rabbit until I can restore - It has a brand new battery in it.
 

I have had problems in the past, leaving an interior light on, lights in
general, and it pulls all the juice out of my existing battery.  I figure A
back up would be nice - so I can listen to the tunes while I parked Too.

Question:

Under driver seat there a wire there - whats it for ??  

Suggested installation.