My speedometer suddenly quit working. What steps do I need to take to figure out the problem?

There is an angle drive at the rear of the transmission, remove the large nut holding the cable, remove the cable and test the cable by twisting the inner cable if there is resistance try the angle drive, needle nose pliers twist the inner flange if it moves without any resistance the angle drive is broken, if that is ok then your speedometer is broken.
Just helped a member here replace the cable today.

With big help and guidance of the almighty @mgphoto we replaced the broken speedometer cable today .

Steps :

1 New GEMO speedometer cable from pelican parts for $50
2 Disassemble center console and harm rest
3 Remove driver seat and carpets
4 Unscrew speedometer cable from VDO and attached twine to end
5 Pull speedometer cable from transmission end all the way out
6 Attached twine to new speedometer cable
7 Pull new cable with twine thru firewall the opposite way it came from, all the way back to dashboard and screw it back inside VDO
8 Test
9 Reassemble everything

Estimated time 2 hrs approximately

Hope it helps

He makes it sound easy.

My speedo went to 0 during a drive today, too. Must be a bad day for speedos. Hope it's just the cable.

Thanks for the responses. I'll start on this tomorrow. Stay tuned.

You can remove the speedometer from the dash and disconnect the drive cable from the back. Have someone hold onto the cable while you drive the car and see if the slotted end spins. If not, the problem is in the cable or the angle drive or inside the transmission. If it does spin (and goes faster the faster you drive) the problem is the gauge.

The cable is the most likely problem.

--DD