Unlike the pilots seat with it's rotary controllers, the front seaters IHADSS brightness is adjusted via the SYM rocker switch on the upper right side of the TDU (the center display, sorry if this is obvious already).
Check that the gun actually has ammo in the mission editor - if you do something like check the 'civilian aircraft' box in the loadout tab, uncheck it, then load a normal loadout, the gun may not have ammo. I can't quite remember the steps I took to do that, but I ended up loading in with zero gun ammo loaded. I think the fault may also occur in the rearm panel in mission as well, but that might have been something I misheard?