1994 GMC C2500 94 GMC 6.5 diesel cruise control problem

You did not mention that you check the fuse for power on both sides, it is fuse #1 on the very top left side of the fuse box, it also runs the window defogger timer, four-wheel drive display illumination, auxiliary battery relay, seat belt reminder, cluster Ignition feed, make sure all of those systems are working. Enable to give you a correct wiring diagram I will need to know if you're VIN is "F" "P" "S" let me know.

You fail to tell me you're truck VIN number, so I will choose the most common one before you remove or replace cruise control module, tried this electrical test first;
1. Turn ignition switch to RUN position. Disconnect vehicle speed sensor buffer connector No. C289. Connect voltmeter from Brown wire at vehicle speed sensor buffer connector No. C289 to ground. If there is battery voltage, go to next step. If NOT, service and repair open in Brown wire at vehicle speed sensor buffer connector No. C289.

2. Connect voltmeter from Brown wire to Black/White wire at vehicle speed sensor buffer connector No. C289. If battery voltage is present, go to next step. If NOT, service and repair open in Black/White wire from vehicle speed sensor buffer connector No. C289 to ground.

3. Raise and support drive wheels. Start engine and place gear selector in Drive. Connect AC voltmeter from Purple/White wire to Light Green/Black wire at vehicle speed sensor buffer connector No. C289. If there is an AC voltage reading, go to next step. If NOT, check for open in Purple/White and Light Green/Black wires. If wires are okay, replace speed sensor(s).

4. Connect vehicle speed sensor buffer connector No. C289. Connect AC voltmeter from Red/White wire at vehicle speed sensor buffer connector No. C279 to Black/White wire at vehicle speed sensor buffer connector No. C289. If there is an AC voltage reading, service and repair open in Red/White wire from vehicle speed sensor buffer connector No. C279 to cruise control module connector No. C 178. If NOT, replace vehicle speed sensor buffer.

5. Ensure GAGES fuse is okay. Turn ignition switch to RUN position. Turn on cruise control. Check for battery voltage at Pink/Black wire terminal of cruise control switch connector and cruise control module connector.

6. If battery voltage is not present, repair open in Pink/Black wire. If battery voltage is present, check for battery voltage at Gray wire terminal of cruise control switch connector. If battery voltage is not present, replace cruise control switch.

7. If battery voltage is present, check for battery voltage at Gray wire terminal of cruise control module connector. If battery voltage is not present, repair open in Gray wire. If battery voltage is present, check for battery voltage between Gray and Black/White wire terminals of cruise control module connector.

8. If battery voltage is not present, repair open in Black/White wire. If battery voltage is present, check for bafteiy voltage at Gray wire terminal of brake release switch connector. If battery voltage is not present, repair open in Gray wire. If battery voltage is present, check for battery voltage at Brown wire terminal of brake release switch connector while depressing brakepedal.

9. If battery voltage is present on vehicles with automatic transmission, go to step 7). If battery voltage is present on vehicles with manual transmission, go to next step. If battery voltage is not present, adjust brake release switch. See BRAKE & CLUTCH RELEASE SWITCHES under ADJUSTMENTS. Replace brake release switch if it cannot be adjusted.

10. Check for battery voltage at Brown wire terminal of clutch release switch connector while depressing brake pedal. If battery voltage is present, go to next step. If battery voltage is not present, adjust clutch release switch. See BRAKE & CLUTCH RELEASE SWITCHES under ADJUSTMENTS. Replace clutch release switch if it cannot be adjusted.

11. Check for battery voltage at Brown wire terminal of cruise control module connector while depressing brake pedal. If battery voltage is present, replace cruise control module. If battery voltage is not present, repair open in Brown wire.

I am trying to send you a wiring diagram but I'm having trouble

That the one selected, so go ahead and hold the procedure. And let me know what happens.