UFO,
The most common glow plug failure is they quit heating up

The second most common failure is breaking them upon extraction. The 3rd most common but rare failure is the glow plug ceramic tip cracking and spitting parts of itself during vehicle operation. I personally think that a ceramic glow plug falling apart in the engine is related to mishandling of the GP during shipping or delivery....just dropping a ceramic plug 2-3 inches onto a work bench is sufficient cause for me not to install one.
I'll probably need to replace mine again sometime this winter, no. 3 resistance is creeping up again. I will use a penetrating oil and spray it on when engine is hot. After several hours when block is cooled down to just above room temperature, I'll start removing them. The pentrating oil is overkill cuz I used anti-seize compound last time I installed new ceramics.
UFO, when you need to replace yours, give me a holler and I'll help you with their removal and replacement. Bring some B100 along too...I think I'm owed several gallons for he hood wind screen but never got around to pick up the remainder.
take care,
Drew
PS All in Colorado, I'm hosting a CRuD transmisson install party this Friday. I've taken it out already and Sir Sam is helping me and my bad back reinstall the replacement tranny. Adult beverages will be served. After the vehicle is made driveable, I'll be celebrating by driving it to the bad side of town to douse my CRuD in gasoline and torching it for the insurance money. Flo from Progressive is invited for the prison conjugal visits after I'm convicted.