Best thing with the cable is pull the inner core out, wipe it clean, then put a smear of grease on it. Leave the top 100mm or so dry- you don't want it getting into the speedo head....!
BTW I've found many new speedo cables are bone dry when you get them..
If none of your gauges are working check you are getting 10V from the Voltage stabiliser can on back of the speedo. Often these old things fail and you get 0V or 12V+.
If it's toasted, PM wombat, he can sell you a new electronic one, cheap.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.