Here is the RIC 381 schematic showing the .0047uF cap.
http://www.rickenbacker.com/pdfs/19502.pdf
Look at my bass guitar drawing below and replace the treble tone pot with the new switch/pot and run the two wires from the switch to the places the .0047uF cap is soldered. The switch will then short out the cap when pulled. You may need to add the cap, in which case pull the wire that is in place of the cap at the treble volume pot end and solder it to the switch. Put the cap in from the selector switch (at the point the wire you just moved is connected) to the volume pot terminal you removed the wire from, then run the second wire from where you just connected the cap at the volume pot over to the other switch contact. Voila, pull the pot up, and you have the cap push it down and the cap is out.
