The brand and model of keyboard may help as some one may have a schematic and layout for it.
However see the attached diagram and maybe can suss it out.
With 6 terminals I expect it to be stereo with 2 identical channels.
Two of the terminals should go to ground the only two connected together.
From the diagram 1 and 1' .
Two should be connected to the "other end" of the pots resistance and have the signal coming from the sound module.
From the diagram 3 and 3' .
Here from 1 to 3 and from 1' to 3' you should be able to measure the pot's or faders resistance.
Turn the pot/fader to full volume and the wipers (the only 2 terminals left) should measure low resistance between 2 and 3 and 2' and 3'.
A high resistance reading (twice the pots value)would indicate measuring between 3 and 3'.
If you now bridge between 2 and 3 and 2' and 3' as long as you always leave the volume at full it should work.
A new replacement fader would be better !