Boss RC-30 looper input question (top tech-head support required!)

oktorockto

Moved from FX board to more appopriat domain.

I have an RC30 on my pedal board and want to use the left hand input for guitar and the right for vocal. I am not using the on-board preamp as it isn't that great so I am using the right hand side of the aux input for the vocal. This also means that the input impedance is matched to my mic pre- output.

I have discovered that if you only have the guitar plugged into the left hand (mono) instrument input the unit acts as if it still mono - i.e., any guitar recorded is output on the L and R outputs. If I plug a guitar lead into the right input (with nothing connected) then the guitar records just to the left side as I want. BUT -  the plugged in lead hums a lot. 

I realise that it will be the internal switching in the right hand instrument input jack that tells the cpu whether to route inputs to one or tho outputs. Does anyone know how I can fool it to do this? Do I just need to connect a jack plug with no cable? If so do I leave the contacts open or just solder a jumper wire between ring and tip? Could I just find something jack plug sized and insert it but made of plastic?

I can see that it is a switch broken by the jack tip from the schematic I just found https://music-electronics-forum.com/attachment.php?attachmentid=53430&d=1556222719 so maybe a dummy plastic jack plug?

Anyone help is much appreciated.

ICBM

You need a shorted plug. (Tip to ground.) that will both operate the switch and mute any noise on the input.