Attach a 12v wire to the speedo end of the old cable using a loop and cable ties and feed it down the chassis tunnel into the engine bay. Then attach the new cable to the wire using a loop and cable ties, and pull the new wire back through the tunnel and up behind the dashboard.
There is not enough room behind the dashboard to feed the long cable through from that end.
If the inside is stripped out, it does not matter from which end you start.