I'd like to go on a diversion called "fun with electricity" for a moment if I may.
There are a lot of similarities between the RX-8 and the MX-5 and this includes the way the cars are wired but if you look closely there are some differences that can trip you up. Example: the thermostatically controlled fans. Here are two wiring diagrams, first from the MX-5 and second from the RX-8.
Background: I installed the radiator from the MX-5 as well as the fan from the MX-5 into my RX-8. The fuse / relay box is from the RX-8. At first glance, everything looks okay. Both cars have three fan relays and both cars have four total connections to the fans (i.e. there are two motors, each with a power and a ground). But upon closer inspection they work slightly differently.
It took me a while to understand what was going on here but here goes. The MX-5 first. There are three fan speeds. Low has half of the windings powered with +12V switched through Relay 1 and the fixed ground wire. Medium is the same, +12V switched through relay 1, but relay 3 is also energized, shorting the second set of windings (presumably higher current) to ground. High energizes all three relays, so Relays 1 and 2 energize the high-side windings and relay 3 grounds the low side winding. Blue is "low", green is "medium" and red is "high"
Looking at the RX-8 diagram, it's a bit different. Relay 3 is actually a SPDT relay. Just wiring in the MX-5 fans with this configuration won't work, because if you look closely, just energizing relay 1 won't give motor 1 a path to ground. Well there is one, but it's in series with motor 2. Follow the green wire from the low side of motor 1 and it goes to the high side of relays 2 and 3. Relay 3's low side connects to the second cooling fan motor. You'll also notice that the RX-8 only has two control wires going to the relays, whereas the MX-5 has three. I really didn't want to add a control wire, and I didn't need to.
What I did was actually very simple. I cut the green wire where it exited the fan motor and connected it to ground. This means when relay 1 is energized, it will drive the first set of windings and run the motor on low speed. Then when Relay 3 is energized, the second set of windings will be powered and the motor will run on high. I've lost the medium speed setting, but I don't see this being a problem.
