Start simple…the rabbit hole that is the climate control system can get deep.
First please understand that there is a difference between British Lucas fuse ratings and North American Buss fuse amp rating standards…essentially, when replacing a Lucas fuse with a US fuse…use 50% of the rating. A 50 amp is replaced with a US 25 amp. Over fusing can heat and distort the fuse holder.
Since you claim the servo is responding to temp knob changes…i recommend you first check for 12 volts at the large brown with white trace wire at the main fan relay pack on the left side of trans tunnel. If you have 12 volts…you either have failed relay, failed fans, failed resistor pack (unlikely) or bad grounds. No voltage…recheck fuse and holder.
You can bridge from the large brown power wire to any of the solid color wires Red, Yellow, Blue or Green slate to bypass the relay contacts and power the fans directly. If they run…I would start with the relay pack.
Of course, the various fan circuits also switch grounds to over ride various speeds. But in all cases…you should have at least LOW fan speed.
The shotgun shell style amplifier is a solid state unit which works fine as long as the servo is in good working condition…as our cars age…the grease in the servo gear box become thick and tar like causing amp draw to increase beyond the max rating of 1.5 amps for the IC chip…generally causing them to crack and destroying the amp.
Best of luck.
Test and report back