The common low/high boost mod is to simply insert a switch in the wire between the amal valve and ECU pin 16. Sounds like on your car, someone has simply connected the amal valve wire via the switch to a permanent earth.

solution for your car :
1 unplug from the switch the wire between the switch and the thicker brown wire (probably an ECU earth)
2 connect a wire from ECU pin 16 to the switch (the original wire from the switch to the amal valve is OK)