I'm studying this stuff now while preparing to install my Mk Viii fan. I ordered the leash dual 70 amp relay module (p/n 91003049). My plan is to hookup a 3-way switch for On/Off/DCC controlled. In between autocross or drag strip runs, I want to be able to cool things off. I think 70 amp motor relays with a 40 amp fuses is the way to go. Apparently, the Mk8 fans spike to 100 amps or so on intial startup. The DCC will tame this on it's circuit. I am worried about the 40 amp fuse being enough for the 'on' switch circuit. I will connect that circuit to the 'Low' input wire to the fan. However, my understanding is that sometimes that's just all the same circuit. Something about an inspection cover on the fan that I don't see.
That leads me to a question: If the DCC is sending power & the "on" switch is also sending power, will that cause a problem for the fan? If so, I'll figure a way to prevent that. I would guess that since they'd both be 12V power supplies, the fan wouldn't care and would run at whichever provides the higher current.
Edit: looks like I'll only need 1 relay for the manual switch. Pin 13 on my DCC controller (FK95) takes an ignition switch signal.