Cannot locate the small metal tube that the clutch cable runs through on its way back to the clutch fork

The cable runs inside the tunnel tube from the pedal back to the firewall where it comes out and attaches with a clamp to the transaxle. If you look up along the gearbox mount where the fuel lines run you will see the cable housing below them. The other end of the tube comes out behind the clutch arm under the throttle linkage.

Steve, my car (when I bought it) never had a clutch cable installed.Therefore,I don't know where the tunnel is.I have a 5-speed and the only tunnel I see is where the shifter linkage runs through.Is that the tunnel?BOB

Yep, that is the center tunnel. If you are under the car and look on the left side of the linkage you should have the throttle linkage and next to that the fuel lines, the clutch cable is below the fuel lines.

Thanks Steve. i didn't realize it went through the "center tunnel" i was looking for a small tube. you guys are the best. thank you. Bob.

It is a small tube but it's sort of hidden. Try this for a guide on replacement with better than oem pictures.
https://blog.fcpeuro.com/how-to-replace-clutch-cable-air-cooled-porsche-911

Thank you Steve for your EXCELLENT info.Take care,BOB.You guys provide a great service,THANK YOU.

You are welcome. Those are fun cars to work on because they lack all the modern electronics.