both accessible behind the cluster? no
the upper one is, remove that and pull it through the firewall
unscrew the lower from the transmission
bring them somewhere that you can hang them up overnight and drench it with WD or even just motor oil, allowing it to saturate overnight and get all the way through it.
If you see it in the morning, and there's oil coming out of the bottom of it, you won.
I made a little system, making damn sure plenty of oil flowed through, out of a plastic cup with a 1/8" or so hole in the bottom of it. maybe a bit bigger. it has to be larger than the plastic nub sticking out of the speedo end. Oil will drip down the nub and into the cable.
Poured about a shot glass worth of oil in the cup and let it drip down the cable through the night.