How do I replace the ignition switch on my 95 Chevy k2500 6.5L Turbo Diesel? And where exactly is the starter relay, I've checked all the obvious places and haven't found it.

The key lock cylinder operates the ignition switch sometimes ignition switch is separate sorry I didn't find where ignition switch is located if the lock cylinder is plugged into it.
You should have a starter relay and starter relay fuse.
This diagram of starter circuit shows the clutch pedal switch in the circuit on an automatic should have the neutral safety switch in the circuit. This is for 95 Rodeo.
Stater Motor Circuit Diagram (1995-1997 2.6L Isuzu Rodeo) (easyautodiagnostics.com)

How do I remove the key ignition cylinder? ' Isuzu SUV Forum (planetisuzoo.com)
Isuzu Trooper Ignition Switch Replacement ' Cost & Service ' RepairSmith
Changing the Ignition Lock: Changing the Ignition Lock, Safley ... (2carpros.com)
Starter and solenoid
Isuzu Trooper Starter replace under 1 Hour - YouTube
Isuzu Trooper (1995 - 1996) - fuse box diagram - Auto Genius
Starting System & Wiring Diagram - YouTube

The fuel relay will activate for 3-5 seconds every time you turn the key on. It should not keep the engine from cranking. I have seen problems with the ignition switch that will keep the truck from cranking. Make sure that the battery connections and clean and tight. Check to see if 12 volts are getting to the starter when you try to crank it. Hold the key in the start position and while holding it, strike the starter with a hammer. If it starts , replace the starter.

Have you checked your ignition switch?They go bad

There is a safety switch on top of the brake pedal lever or clutch lever which is suppose to prevent lurching on start-ups. It is suppose to insure that the Brake pedal is pushed OR the clutch is down before the engine will crank.

The Ignition switch is interrupted by the above cutout switches. But when Ignition is turned on, the Glow plug timer should click and cycle if Power Relay is giving it juice. So if you do not hear that type of clicking, the glow plugs will stay cold.

I would check to see if power is going to the Power Relay above the glow plug timer, and if it is getting power, does the Power Relay send it to the Timer? The starting circuit is more involved in a Diesel because the fuel must be atomized by heating it for initial starts. But you must be getting this process if jumping the solenoid results in the engine starting.

Therefore the big test is whether the small wire to the starter solenoid is dead when a foot is pressing the Brake or clutch pedal when the Ignition key is in the STARTER position.

So either your Ignition switch is bad at the STARTER terminal position or the signal is interrupted by the anti-lurch Interlock switch. It is quite common for problems with this switch being hit and broken or displaced by the Drivers' feet.

There was also no indication of whether you jumped the solenoid on a warm engine which could atomize the fuel by compression or a very cold engine which needed the glow plugs to work.

Please comment on my info and report your success.

pity you did not get a load test on the battery first
dead cells or bad terminals will do what you describe

It sounds like a bad fusible link. Hard to say where it is. Could be near the starter solenoid or the fuse box under the hood.

ignition relay probably stuck...could be an oddball short between ignition relay signal wire and another hot wire, but highly unlikely...replace relay

Problem may be the Crank Case Sensor. Get this checked before changing anything else.

http://www.youtube.com/watch?v=RuIislTGOwA

1. check your battery.
2. check your connection to the starter
3. check your ignition key.
4. check your relay, etc

some are fused and has relay. check fuse and relays. replace with the same type, you could try to interchange the relays if they are of the same type.