Power supply to Sirius Radio

yes, some sirius radios work on 5 volts, and some work on 12 volts. Your sportster replay is a 5 volt unit. If you trace that wire back, you will find a cigarette lighter plug with a transformer built into it, with either wires soldered to the terminals, or a female cig plug wired up behind the dash. Whatever sirius unit you plan on hooking up, I strongly reccommend using the transformer that it came with......however if you're willing to take a chance, it's most likely 5 volts.