I did this last night for a late model with the 2-piece cable. It really wasn't that bad. Get the car up on 4 jackstands and you may want to remove the steering wheel as you'll be spending some time crawling under the dashboard.
My approach was a little unconventional, but it went pretty well. So with that said.....
The best way to do this is feeding the cable forward, from the dash back to the trans. The metal end of the speedo cable can be fished through the passages, the plastic end cannot. Also, when feeding through grommets, you must get the grommet out of the hole first. The cable will NEVER fit with the grommet in place. Once the cable is through the hole, push the grommet back into place around the cable.
To make routing a little easier, I installed just the sheath, then slid the inner cable in after the fact. You may want to do the all-in-one approach.
Start by removing the little grille in front of the driver's side of the windshield, then dislodge the grommets in the bulkhead for the brake/clutch lines, and also the grommet just below, also for the brake/clutch lines where they head down into the cabin.
Feed the metal end of the cable into the space at the base of the windshield, then through the bulkhead (split grommet) and pull a few feet of cable through.
Now feed the metal end through the round grommet into the cabin. Note that if you go straight into this hole, it will hit the brake/clutch MC hardware. You must angle this cable so it heads towards the center of the car. Once you can fish it through and get it headed somewhat towards the heater core, feed a few feet of cable into the cabin.
At this point, you want to set the location of the plastic end of the new cable. It should be just under the base of the windshield. Hold that in place and feed all off the excess cable into the cabin.
Crawl under the dash and fish the speedo cable into the hole in the floor. This hole is about 1" in front of the throttle pedal, outside of the tunnel. There is a funky split grommet below the car that should be removed for this step. It was a little awkward to get the cable started in that hole. Once the cable is through, feed as much of the cable as possible into that hole, then secure the cable to the buklhead with the existing plastic clip, above the end of the tunnel.
Under the car, clip the new cable in along the tunnel. There are 6 or 8 stout plastic clips for this purpose.
There is a bracket near the bottom of the fuel tank that also secures the cable. Pop out the split grommet and feed the cable up, then replace the grommet.
Arc the cable over the trans & clutch arm. Pull the inner cable out a bit and engage the speedo drive gear, then slide the sheath down and thread on the connector.
From the glass lined tanks of old Bertone