Have you tested them yet, as described in other posts? They do not go bad from age.
If you decide that you absolutely have to take out the glow plugs, do these things in order: 1) get a compressed air hose and nozzle to the engine bay and blow out all the crap that has collected at the base of the glow plugs. **To locate the glow plugs, find the eight wires with spade connectors going from the back of the block to each cylinder between the injectors and intake manifold (that means one wire per cylinder, not 8 per), disconnect each wire carefully and set aside.**
2) get a 3/8 inch deep socket and ratchet or bar (I have used this, you may need a 10 mm socket).
3) slip the socket over the glow plug hex and remove carefully. They are right hand threads. ** important note** the GP threads are METRIC. Do not mess them up unless you have a 10mm tap (I don't know what pitch they use; there are several) ready to go. If there is resistance, the heat coil on the end may have exploded and will not come out easily or at all.
4) inspect the plug and test again for resistance. Another easy test is a battery charger with a charging meter. Connect the minus clamp to any part of the body (the threads are preferred- if you clamp to the heater element and the plug is good, there will be considerable heat generated), the plus clamp to the spade connector and turn it on (also for no more than 10 seconds). If the meter shows any movement at all, the plug is OK. If there is no movement, the plug is burned out, no internal circuit, no signal to meter.
5) repeat for each plug and replace as needed.
How much experience do you have working with a diesel engine? They are very little like a gas engine in the way they actually make power. No ignition or spark plugs, no carburetor, no throttle plate. The good folks on this forum are very patient and helpful. I know, I am very new to working on diesels myself. But you can learn fast here, everyone eats and breathes diesel.
Cheers!
Sorry, as I was writing my response, three other posts were made.