Generic Electronic module.

Check body grounds, check the wiring to the inst cluster, and check the body control module (BCM) lets check the gauges fuse as well.

You should look for a qualified repair shop , unless you know how to test automotive electrical circuits . Battery drain or parasitic draw on the battery is not easy to find if not familiar with automotive electrical testing . Also having wiring diagrams an service repair info . Does the truck run an drive ok ? The speedo an tach not working could be a problem inside the instrument cluster or VSS - vehicle speed sensor for the speedometer .
Vehicle Speed Sensor (VSS)
On vehicles equipped with 4-wheel anti-lock brake systems (4WABS), the vehicle speed signal is generated by the rear anti-lock brake sensor and sent to the 4WABS module. The 4WABS module sends the vehicle speed signal via circuit 679 (GY/BK) to all systems that require a vehicle speed signal input. Vehicles equipped with rear anti-lock brakes (RABS) generate a vehicle speed signal from the rear axle speed sensor. The generic electronic module (GEM) receives this signal (VSS_GEM) for internal use and then distributes it to the appropriate other users (i.e., the powertrain control module [PCM], speed control module, and speedometer).
Speedometer
The electronic speedometer (17255) receives a speed signal from the 4WABS module (if equipped with 4WABS) or the generic electronic module (GEM)/central timer module (CTM) (if equipped with RABS).

Tachometer
Gasoline vehicles are equipped with a 6,000 rpm tachometer and diesel vehicles are equipped with a 4,500 rpm tachometer. The tachometer is hardwired to the PCM through circuit 648 (WH/PK).

https://www.youtube.com/watch?v=efKoWifdcag&t=844s

https://www.youtube.com/watch?v=6xFo_oHFF48&t=2056s

Instrument cluster - stepper motor inside that controls gauge . could be the problem , GM instrument cluster gauges stop working all the time . Plus could be other thing's , wiring , module's etc.... Just depends where the tach gauge info. comes from . Hooking up a factory scanner an checking data to the cluster from the PCM/ECM -engine computer . The instrument cluster has a microprocessor inside an it's hooked up to the class 2 serial data buss .
Tachometer
The instrument panel cluster (IPC) displays the engine speed based on the information from the engine control module (ECM). The ECM converts the data from the engine speed sensor to a 2 pulses/engine revolution signal. The IPC uses the engine speed signal circuit (2 pulses/engine revolution) from the ECM in order to calculate the engine speed. If the IPC detects a malfunction in the engine speed signal circuit, the IPC uses the serial data message from the ECM indicating the engine speed.
You may wan't to take it to a qualified repair shop.

Could be a lose connection in the dash , more then likely . Who replaced the gauge an drive indicator ? The horn / cruise control /turn signals / radio all share a common ground point it looks like ,G203
G203 Behind upper LH side of I/P, near cowl
Maybe after gauge & drive indicator replacement connection became lose . None of these componets share a power supply (fuse) I can't send picture of the location of G- 203 sorry ! Instrument cluster would need to come out to see the ground .

Check wirring for damage

tach is most likely a cam sensor, hvac could be a bad vacuum problem (bad hose ) the electrical (dash stuff) I would start looking for a loose or bad ground wire or dirty battery terminals

Dirty Power and Ground connections. Working from the Battery, disconnect and clean all joints, along the Harness to the Main power box, and do the same in there.
It is a Ball acher, but it will fix the problem.
Regards, DT

Try disconnecting battery for couple minutes and see if it starts working again I have done that in the past

I am not sure if this right and there should be diagram on the back of cover.


1 5A Power Mirror Switch

2 10A Daytime Running Lights (DRL),
Back-up Lamps, Transmission,
Passenger Air Bag Deactivation
Switch, Blower Motor Relay

3 7.5A Right Stop/Turn Trailer Tow
Connector

4 — Not Used

5 15A 4x4 Control Module

6 — Not Used

7 7.5A Left Stop/Turn Trailer Tow
Connector

8 — Not Used

9 7.5A Brake Pedal Position Switch

10 7.5A Speed Control Servo/Amplifier
Assembly, Generic Electronic
Module (GEM), Shift Lock
Actuator, Turn Signals

11 7.5A Instrument Cluster, 4x4, Main
Light Switch, Truck Central
Security Module (TCSM)

12 — Not Used

13 20A Brake Pedal Position Switch

14 10A or Not Used 10A: If equipped with Anti-Lock
Brake System (ABS) Control
Module

15 — Not Used

16 30A Windshield Wiper Motor, Wiper
Hi-Lo Relay, Wiper Run/Park
Relay

17 20A Cigar Lighter, Data Link
Connector (DLC)

18 — Not Used

19 25A PCM Power Diode, Ignition, PATS

20 7.5A Generic Electronic Module
(GEM), Radio

21 15A Flasher (Hazard)

22 20A Auxiliary Power Socket

23 — Not Used

24 7.5A Clutch Pedal Position (CPP)
switch, Starter Interrupt Relay

25 — Not Used

26 10A Battery Saver Relay, Auxillary
Relay Box, Restraint Central
Module (RCM), Generic Electroic
Module (GEM), Instrument
Cluster

27 — Not Used

28 7.5A Generic Electronic Module
(GEM), Radio

29 20A Radio

30 — Not Used

31 — Not Used

32 — Not Used

33 15A Headlamps, Daytime Running
Lamps (DRL) Module, Instrument
Cluster

34 — Not Used

35 15A or Not Used 15A: Horn Relay if Not Equipped
with Truck Central Security
Module

36 — Not Used

4 resistors on the back of cluster bad get them at radio shack.