my first FRS had the upgraded headunit which came with RCA connections on the back, damn it was was so easy to hookup everything to my own amp/sub...I got the upgraded headunit on my 2nd FRS but they changed it so it no longer has the RCA ports
so your only options are to get different HU or get some toyota wiring harness and connect through the wiring that goes to stock amp in the trunk..
this is how I hooked up mine..