93 riviera instrument panel no longer shows any gauges, speedometer, tachometer. But does show both turn lights steady on, hazard, seatbelt, brake, etc... all steady on. Any ideas on the cause of the problem?

You might have to replace the speedometer cable

Check engine light ,abs , air bag lights etc.... come on when key turned on ?
Fuse IPC/HVAC BATT 10 amp located in the left IP junction block . Do you know where this junction box is ?
Does the heater an A/C work ? If so the fuse isn't the problem .
Speedometer
The speedometer is a road indicator. The vehicle speed sensor (VSS) is a permanent magnet generator which is located on the transaxle. The VSS sends an input to the PCM whenever the transaxle differential carrier turns. The PCM then buffers the VSS signal. The PCM transmits the VSS signal to the following components at 4000 pulses/mile through CKT 817:
• The instrument cluster
• The cruise control module
• The radio (uplevel)
The instrument cluster uses the input in order to indicate the vehicle speed on the speedometer.
Instrument Cluster Indicators
The BCM controls the instrument cluster indicators through the SPI serial data communication bus. Refer to Body Control Module System Check in Body Control System.

IPC/BFC ACC
10A
Instrument Cluster and Body Control Module (BCM)
IPC/HVAC BATT
10A
Instrument Cluster and Heater A/C Control Assembly

Cold solder joints were a common problem with GMC instrument panels of that time. I've found surface mount resistors just rolling around inside panels with some just plan gone. If so you can get a panel from junk yard remove parts from that & fix yours. Removal & repair is not all that hard.

Dash board or gages

first, you need to unscrew a lot to remove your gauge box (the one that contains all instrument lights, speedometers, tachometers, fuel gauge, etc). then, after removing your gauge box, you can see at the back side the location of the bulbs.

You will have to replace your main computer module but can get a rebuilt one at dealership.

try resetting the service lights

there are 2 large terminal plugs with the wiring for all the instruments that are plugged into the back of that panel. make sure they are snug and in place. You reach in and under but it may easier to "simply" remove the cluster. 4 screws hold it in place accessible form underneath and with a long phillips screwdriver and lighting.

It could be the voltage regulator for your instrument cluster is the problem.

check for blown fuse for gauges,light is a pro blem code,thatll have to be scanned to point at problem area.