If a 3.5mm stereo cable can't control the
front panel meter illumination, maybe you can add a
3.5mm Mono Plug to 3.5mm Stereo Jack Adaptor
on REF3 12v trigger output.
https://www.amazon.com/Monoprice-107128-Stereo-Adaptor-Plated/dp/B002N1XMPO/ref=pd_sim_23_2?_encodin...
https://www.amazon.com/Monoprice-107128-Stereo-Adaptor-Plated/dp/B002N1XMPO/ref=pd_sim_23_2?_encodin...