When I installed the running lights I walked around the truck to check all the marker lights. Much to my chagrin one of the sunvisor marker lights was burned out. While I stared at them another flickered.
Unfortunately, there's more to it than just popping in a new bulb. The marker lights are a sealed LED assembly, P/N 85107907. I ordered a set of five on Ebay for $60. One is bad. One flickers. Might as well replace them all and be done with it.
There is a cover that has to be removed that is held on with four T-30 screws. The lights themselves are held in place by clips. It's not that bad of a job except for reaching the center lights.