You might want to look at this thread.
RX-8 Schematics?
The wiring schematics were posted here and it looks like the top section of the schematics pages shows the wiring coming from the "Audio Unit" which I assume means the head unit.
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2004 RX-8 Titanium, 6-spd, black/chapparal, GT pkg, no nav
1991 RX-7 convertible