I've got the workshop manual here, only thing we need to know now is you engine ECU version:
M.2.10.3, fan controlled by a thermal switch in the radiator
M.2.10.4, fan controlled by MOTRONIC ECU
In the first case, the relays could be knackered or the thermal switch
In the second case the relays or the ECU temperature sensor are knackered.
First case, just pull out the correct relays and check them with a multimeter. Similar with the thermal switch.
What you can try with the second option is disconnect the ECU's temperature sensor. In that case the fan(s) should stop (Temp sensor is NTC, so no sensor mimics a low engine temp. -no fans)
For the relays, and both ECU types, there is one for every speed of the fan(s) and they should be found at your fusebox; i99 is low speed, i100 is high speed.
And last but not least; gtv-workshop-repair-manual
. Should give you all that you'll ever want to know -about 916's that is.