Seeburg SCD-1 Jukebox Muting Circuit?

[Majoraslayer]

I received this jukebox from my aunt a couple of years ago.  Recently the CD changer stopped working in it, which really sucks since parts seem impossible to find and the 6-speaker sound system is amazing.

To keep this thing alive, I'd like to enable it to play sound via the RCA input.  This particular model has an aux input, but for some reason it only plays the sound from one speaker.  For it to play correctly I'll need to connect to the CD player's input on the amp board, which conveniently also happens to be RCA.  However the muting circuit seems to be preventing it from playing.

Does anyone know how I might get the amp to always stay unmuted?  According to the schematic I found, plug J3 on the amp board is labeled mute.  I tried disconnecting the wire but it didn't make a difference.  If that wire is the key, that means either it needs some sort of voltage applied to it to unmute  (which I can't check since the non-functioning CD player prevents it from sending the signal), or the pin needs to be grounded.  I'm not great at reading schematics, so if anyone has experience with doing something like this I could really use some help.