Look for grounding issues. That's usually what's wrong when this happens.
If it's passive electronics, the problem should be easy.
Look for a wire that's not apparently connected to anything. That's probably your ground wire. Most basses have the ground wire smashed under the bridge plate so that it's all grounded to you.
You may also solder it to the ground side of your jack.
Then you have to make sure all the ground wires from each pickup and pot are all connected so you don't have a break anywhere.
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