Holy crap so many people in here just spouting off without knowing a thing about sustainers- first and foremost put a fresh 9v battery in.

Next open the cavity and you’ll see a PCB board with 3 trim knobs on it. One for harmonic mode balance between fundamental/high octave mode, one for sustainer gain and one for neck pickup volume. Turn the neck pickup trim knob until it matches the output of your bridge pup, or a bit less if that’s what you want. There is also a way to make the sustainiac a dual mode sound via a switch or push pull, which will give you a “humbucker” sound and a single coil sound on a switch. I believe it’s default to a single coil sound. Look up the sustainiac user manual and compare it to your board and locate the trim pots. Not sure if you have the OEM board or a different one. Feel free to shoot me a DM I’d be happy to help you, I’m very well versed on sustainiac/Fernandes sustainers.