twin neck selector switch

[Dom in Dorset]

I'm building a twin neck fretless/fretted.
It's got a P bass pick up and a piezo bridge for each neck.
I've been trying to put a switch in to select between top neck/both/bottom neck withy no luck.
I wired each half like a jazz bass with the piezos in place of the neck pick up, this worked well in isolation (only tested with one string) but when the switch is put into the circuit I get awfull buzzing if only one neck is selected and although it can be played with the switch in the "both"position, you can't turn the volume right off on the unused neck, also the tone controls seem to influence the tone of the other neck!
The switch was fitted by taking the earth side of each output and connecting it to the switch, a common wire runs to the jack from there.
I know nothing about wiring, but I can copy a diagram, what's wrong?

[BOD2]

I'm not quite clear about how you've wired the selector switch - you say you've taken the earth side of each output to the switch ? If so, then that might be the problem.

All the earths should be "common" and all connect to the same point, which is ultimately the earth side of the output jack.

What you need to switch is the "hot" wires from each neck's output and connect these via a 3-position switch to the hot side of the output jack.

[Dom in Dorset]

Sounds like it might be worth a try, I'll reprt back.

[Dom in Dorset]

Thank you BOD2 it worked!
I'll be posting pictures in build diaries ASAP