I ordered my replacement cable and firewall grommet from USA Part Supply (800 872 2031 or www.usapartssupply.com. They were about $23.00 for the two items in May of 2000.) I believe the cable was made in China, so you may want to shop around for a GM or other domestic brand. If you find one, let me know the source and cost and I'll post it here.

1. Put the front wheels up on ramps to give you some safe room under the car. Before you root around behind the dash, you should disconnect the battery. Put that Rolex someplace safe, too.
2. Under the car: Unscrew the cable connection to the transmission (see illus. #1). Disconnect the cable from any frame clips or other restraints while under the car.
3. Inside the car: Remove plastic plate at the base of the steering column (3/8" socket-head screws - see Illus. #2).
4. Work your left hand up behind dash (there is a fairly decent route near the fuse panel. Unscrew the speedo cable at the back of the speedometer (see illus. #3). My original cable unscrewed fairly easily after 32 years of service, using a thumb nail to get it started.
5. Back under car, pull cable down through the firewall and discard.

Illustration #1 - cable connection to driver's side of transmission, near rear.

Illustration #2 - plastic plate at base of steering column, behind brake pedal.
(Pardon the dangling wires from my unfinished cruise control install.)

Illustration #3 - back of speedometer (picture of spare unit).

1. Starting under the car, route the new cable (after following the manufacturer's direction regarding greasing it) through the firewall, pass it along and above the steering column.
2. Inside the car: Shove the new grommet over the speedometer end of the new cable. I found it a very tight fit, but facilitated things a bit with some conduit lube (handy stuff to have around).
3. Your left hand will use that same opening near the fuse panel to work its way near the back of the speedo. Use your right hand to push the cable up along the steering column, while guiding it to the back of the speedo with your left. Feel around for the back of the speedo first to get an idea of where you need the cable to end up. You should be able to find the wiring harness connector fairly easily. The speedo cable connection is just below that.
4. Once the cable is reconnected to the speedometer, remove any slack by pulling gently on the transmission end. Push the grommet into place in the firewall. Replace the plastic cover.
5. Route the cable under the car, clipping it to the frame, if its length permits. Keeping it above the parking brake cables will help keep it away from the exhaust system.
6. Reconnect the larger end to the transmission. You're done!