http://www.ft86club.com/forums/showthread.php?t=23314
The guide linked in this thread really helped me with my amp and sub install, however, the 2015 stock Scion headunit didn't have RCA outputs so I had to buy a line-out converter and splice into the front speaker output wires. I found out which wires to splice using the wiring description from this post:
http://www.ft86club.com/forums/showthread.php?t=9497
Hope that helps.