Prolonged Discourse:
I went to run an errand, but I never left the driveway. After the first failure I immediately tried the ignition key again. Nothing happens. Followed by a useless third attempt at cycling the key. Followed by alternating feelings of “WTF's happening” and “Oh, S**t. This is gonna be expensive.”.
Out comes the multimeter. 12.83v “That's good,” I think. [←- My first mistake. One that would cost me several hours.] All four connectors are tight and without corrosion. I open up the PDC; test & switch several fuses & swap relays. Everything looks good & has continuity. I test the passenger battery. 12.81v [it's an unworthy test (the two batteries are hooked in parallel, after all), but I'm trying anything to get more information] I head towards the fuse panel by the steering wheel in the cab. By chance I depress the power lock switch, & they work!!! Lock; unlock. Lock; unlock. I go to move the power seat forward. Nothing happens. Now the locks have stopped responding too.
Confusion starts gaining ground in my mind, and anxiety is not far behind it. I cycle the key. Nothing happens. I retest the driver side battery. 12.81v I haven't heard any noises that would lead me to believe some circuit had arced & thereby ruined continuity [but continuity to the entire truck?? & through different electrical activities??]. By the time I actually test fuses, I check continuity only & forget to test voltage [top row & bottom row should have constant power]. All that are tested are good. I am baffled, so I head inside to browse the internet. Time passes. Articles & forums get open/closed. Some folks have similar issues, but I don't come across any CTD's with my exact symptoms. More time passes. A friend arrives, & his vehicle runs off of a known good battery.
My fortune will now change, because in the process of hooking up a known good battery, I must disconnect the parallel setup that's been 'hiding' the real culprit – a weak battery. I attach the jumper cables to just the corresponding pos/neg terminals, bypassing my batteries. All electrical functions come to life. Just for grins, I attempt to fire up my truck. The starter makes several clicks before I release the key knowing that the donor battery just doesn't have the juice a CTD requires.
Driver's side battery, now detached from the truck: 12.83v Pass. Side: 12.40v I read somewhere that an automobile's 'resting' voltage of 12.4v → 50% of the rated capacity. I did more tests. Now I hooked up just my good battery to the pos/neg terminals – via the jumper cables [as my batteries are still removed from the truck] – viola! All electrical systems work. I hook up the weak battery – also via the jumper cables – no love! The parking lights won't even turn on! I measure the voltage across the ends of jumper cables… 11.82v ←- this is with nothing attached to the cables. Weird! So, I put the jumpers on the good battery & measure from the end of the cables: 12.81v That's more like it. I've never encountered such a voltage drop over essentially nothing before. That battery is gone.