Actually I believe for the OPs Positron shifter she/he needs a special cable that is like a solid wire rather than a flexible multistrand cable. I also believe it comes with housing as unit.
OH my mistake you can get just the inner wire but they are pricey.
Vintage Shimano Schwinn Positron Inner Gear Cable Bike Bicycle 5 6 10 12 Speed | eBay
$19.99
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Last edited by Bianchigirll; 04-19-15 at 06:54 PM.