I just replaced #4 (pass side rear) this morning. I replaced all of them at 85K miles and this one now at 117K.

There's definite corrosion damage from looking at the old plug. I just can't seem to source where the moisture is coming from. I've wrapped the ac lines so condensation won't drip. I just wrapped the heater hoses that run over that bank of cylinders. I've always checked them when the truck is running and don't see anything.

I also used dielectric grease last time, but water still seemed to make it's way in there. I don't spray under the hood nor do I play in deep water, it's just a daily driver.

Thanks!

 

Be sure the area is dry then play a hose on the front of the truck with the hood closed.
There may be a gasket leak or a entrant point from outside.
Good luck.