The only thing I would be concerned about is checking what happens if both the high and low speed are selected at the same time.
Selecting both the high and low at the same time isn't an issue since one of the two wires you ground for the high-speed setting is one of the wires used to trigger the low-speed. If a person hot-wired only the low-speed wire and the temp rises, the ECU will request cooling and start the low-speed circuit, since that is already running nothing will change. If the temperature continues to rise or the A/C is requested, the ECU will request high-speed and the fan speed will increase. In other words, the ECU will never notice that the system has been hot-wired and it will continue to function as it did from the factory.
, it doesn't matter where you make the connections to ground the circuit, I would choose a place that is easy to get to, protected from rain or road grime, and also easy to route the wires to whatever you're using for a switch. I don't believe that you'll need any sort of relay since it should be a low-current switching circuit (the factory relays will continue to do the heavy [current] lifting). Even though it's a ground circuit, make sure to do it right, solder connections if possible, heat-shrink exposed wires, use dielectric grease for terminal connections, and secure the wiring to prevent chaffing and vibration. Good luck and we look forward to seeing your final result.
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