When I needed to replace the reverse light switch in my 2004 MT, I had a hard time finding an in-stock part at Mazda. From elsewhere on this site, there was indication that parts from a Lexus fit. So, to add to that knowledge, the part I used was a FACET 7.6162 Reverse light switch. It has shorter wires and a different connector than the one on the RX8, so the wire pigtail needs to be spliced on. The Facet part was also a lot less expensive than the Mazda part.

Here are a few images:

FACET 7.6162



RX8 Is longer



After wires spliced.