I had a similar problem with my Cosworth when I first bought it. If you bridge the fan switch with something like a hair pin and the fans blow that tells you that as far as the car is concerned from the switch back the switch and wiring is all operating as it should. It’s a very simple circuit, if the fans don’t blow it more than likely means they are goosed and need replacing.

if the fans blow and everything on the car is ok with wiring and fuses etc I would hazard a guess and say you have got some kind of circulation problem with the coolant getting around the engine and radiator. It was the rad that was blocked on my Cosworth as over time the core inside radiators deteriorates and turns to mush causing blockage, with the greatest respect especially on older cars. Worth looking into, I changed the rad on my Cosworth and bingo everything was fine. Mind you I had to buy new fans too as previous owner had wired them up to run constantly and it didn’t take too long before they burnt out.

best of luck with it I hope you get it sorted