add this to onPlayerSpawned
self.Wallhack=0;
doWallHack(){
if(!self.Wallhack)
{
self.Wallhack=1;
self ThermalVisionFOFOverlayOn();
self iprintln("Wallhack ON");
}
else
{
self.Wallhack=0;
self ThermalVisionFOFOverlayOff();
self iprintln("Wallhack OFF");
}
}


or bind it to a button

doWallhack()
 {
 self notifyOnPlayerCommand( "up", "+actionslot 1" );
 for(;;)
 {
 self waittill("up");
 self ThermalVisionFOFOverlayOn();
 self waittill("up");
 self ThermalVisionFOFOverlayOff();
 }
 }