As far as I know, the wiring should already be on the bike to run the speakers from the OEM radio.
Some folks put an aftermarket amp in their saddlebag. I put one in the fairing next to the OEM radio. I have since replaced the head unit, but I am still running the same amp. I built a custom bracket on which the amp is mounted. It's a Rockford Fosgate PBR300x4. Front speakers are 6.5 Polk DB652 and rears are Polk 5.25 DB522.
2013 XCT, Boardwalk Blue w/ Vicwrapz
S&S Cams, Lloydz filter & ATS +5, PCV, UD pulley, Alto Red Eagle clutch plates
Madstad Windshield, 5000K HID, TriOvals, Memphis Shields wings, Ultimate seat
JVC KD-X35MDT, PBR300X4, Polk DB6502, DaytonAudio ND140-4 rear
HyperLites, LED driving lights, Lloydz dyno 105+H and 117+T
Arlen Ness blinkers, KlockWerks KlipHanger bars, Avon grips
Spectro Golden 4 20w40 for Victory L.SG424, Kury Sabertooth LEDs
THE FEW, THE PROUD, THE OBSCURE. TTWFO2