Amp w/ USB and remote: which one? best way to wire

[CoryM]

Hey guys. I'm not very knowledgeable about car audio so if you can help that would be great.

I'd like to have to ability to play music from a USB flash-drive but I don't want an aftermarket head unit, nor any clutter of aux wires. I see you can get small amps that have USB input, and a remote. I would like to hide one somewhere, and use the remote to control during normal use.

Things I need:

-12v power input
-USB input
-remote (volume, skip, ability to shuffle)
-no draw with key off (I've removed too many crappy amps from peoples cars because of draws)

Stock Yaris stereo sounds fine to me so I don't need great audio, and don't listen to it loud so probably don't have to worry much about distortion.

Questions:
-Do these remotes typically need line-of-sight to function?
-What is the best way to tie into stock speakers or system? Just cut/splice speaker wires? I can take apart the HU and solder into the AUX input from behind as well if that is viable (would that work? amp output to AUX input of headunit).

Car is a 2009 and it does have AUX input on front.

Thanks for any help.
Cory