inline splicing for extra battery power

I am new to the grid and have been following everyone on all of the vectrix blah blah. I just got my "NEW" 2008, and found that driving this to work, I'm short by "THAT MUCH!"(maxwell smart). What I came up with, is I added a nice hitch design to the back and under the tail for pulling a tiny trailer battery power-pak. WOW! IT'S COOL! I noticed that everywhere, everyone talks about the weight, why not pull it, it's the same as a second rider. So what I need from you all is some help in where to tap/in-line splice, a connection for this prototype trailer pwrpak.