With all the work necessary at getting to the GP's you would be well advised to replace em all at the same time.
To test them one could use an Ohmmeter with the low scale looking for resistance only a couple Ohms for a good GP, disconnect the GP timer and test from there. Or use the car battery to momentarily apply voltage and verify that they indeed get hot (watch out and don't get burned in the process!)
Oh, yes, replace the GP fuse, located under the top cover on the timer module I believe on that model. It tends to get hairline fractures that cause the timer to lose its voltage. Only about $1.50 from the Partsforum. I keep a spare in each of my diesels, having seen them fail in that wierd way (its called metal fatigue, common in high current fuses.)
Dieseldiehard
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 295530
1983 300D 243280
1985 300TD 217300
1987 300D 258230

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body. '87 300D w/ Sportline suspension. No more '79 300TD w/ 617.952 engine at 367,750, it was my annual driver to Texas and I sure don't miss it poor A/C heater system!