Personally, I'd be worried about someone breaking in and stealing it, but assuming you've already considered that...
+1 on the headphone-to-RCA adaptor to plug it into the amp.
USB is a 5-volt connection, not a 12-volt connection, so you would need to step-down the voltage (probably using a potentiometer) to get it down to 5V so you can charge your tablet's lithion-ion battery, which charges as high as 4.1V and drains to ~3.6V. If you look at your tablet's charger, it also outputs 5V. If you don't know how to wire a potentiometer, look that one up, or google yourself an electrical engineering lesson in voltage dividers.
As far as where to pull the power from, I would splice into the cigarette lighter wires. I've considered doing that myself. It all stays behind-the-scenes.
1997 Sentra GXE, GA16DE, M/T