i've just been through something similar on my 864 which had rot (solidified fluid and pollutants) throughout the aux circuits. while i've still got one problem (will create a separate thread) i can tell you that the spool valve can be stuck, the solenoids controlling the spool valve can be stuck or dead, the wiring harness can be damaged, and the thumb button(s) can be broken. also the oil cooler can be packed with s**t. i started with zero function and after all of that was addressed (all of it was true for my 864 since the previous owner had never used it and had done other wiring harness "work" without then testing the effect on the aux circuit), i have limited function now. it sounds to me like you have a bug hunt ahead of you. recommend getting the factory service manual if you don't have it yet. so, yes, there is a relay or something, but that's not the only way you can get zero aux power.
