when I did it once it was on an old volvo, because I converted from a fan on the pulley to an electric fan, where there was no place in the radiator for a thermostat switch thingy (I knew you could get aftermarket temperature switches, I just never got around to buying one)
whenever it was on the switch got red hot and I quickly learnt that it should be through a relay (which yours probably is as well at a guess)
you say it's an 'override' but does the fan still come on by itself if you leave the switch off and get the engine hot
(i.e. is there something wrong with the original switching mechanism / sensor / wiring, however it works in the MPI)
Edited by stuart bowes, 23 October 2022 - 02:37 PM.