Based on your 1st post, you said the starter just spun over & over but engine did not turn over from what I got. 1) that don't mean your starter is bad because it's doing what it suppose to do. The "thing" on top of your starter in not a relay--its the "starter solonoid". If the truck is orig. MV the ONLY things that are 24v is both the starter and the solonoid. The starter should have "24V" stamped either on the outside body of the starter or one of the end plates where you can see it. Also the "solonoid" has it stamped on the outside of it.
If the starter is spinning and the engine is not turning over- most likely you have a bad 24v solonoid. Your other problem about the starter continuing to try to crank after you have turned your iginition key to OFF is as said is the "RELAY" under your dash panel next to the big round electrical dianostic conn. Whats happens is these 25 yr old relays cordode and the conn. points in the relay sometime begin to "stick together" when energized. IF IT EVER DOES IT ONCE. EXCHANGE IT. It is nothing special, take it to one of parts dealer and tell its a AC blower relay for a late 80's model Chev. DON"T mention anything about being a military 24v truck===It will confuse the heck out of them.
Also to clear up , I hope, when you turn the ign key to on & start position, all it does is push a 1/8" steel wire rod down so that it hits two contact points located 1/2 way down the steering column. This in turn makes a 12v elec. connection to the 12v relay . This makes a 12v connection to the solonoid on you firewall . At this point it connects the FULL 24V wire going to your solonoid. That engages the bendix inside the solonoid to pull the gear into the flywheel which turns over engine. As for as glow plugs being BAD. why. To me the easier way to check them is to take out using 10mm socket and use a 6-10" jumper wire and use vise grip pliers to hold one end of wire to neg side of 12v batt and (Ijust use my fingers) and wrap the other end of wire around the "spade" part of th GP and hold it tight or you will get a little burning sensation, then I touch the thread part of GP to the pos side of batt. If its bad, nothing will happen. If its good almost immediately you will notice the end of GP heating up and in about 3-5 seconds it will be glowing. It's OKAY and be sure to lay it on metal or concrete so it won't mess something up. Also it may take ten minutes or more to cool down enoungh for you to touch it. While its out look at end to make sure its notstarting to swell. This episele it long enough. Hope it helps.
this can't be happening again. By the time I got the hood opened and the pos and neg wires off I had a melted starter.