I have no idea what to call this component so hopefully this will register with someone 
I have a 2002 Civic EX. The rubber-covered nubby plunger thing that sits in the bottom of the driver’s side door jamb has broken such that it is recessed into the jamb. I can’t pull it out, I think it may have broken off its assembly.
This is a problem because this device reminds me if I’ve left my headlights on. I’ve drained my battery more times than I can count because of this 
I’m just trying to get a sense of whether this is a trivial thing to replace or does it involve disassembling the whole door to repair?
Can this be DIY’d or not attempted?
If the switch is inside the door then the inside door panel will most likely need to be removed in order to get access to the switch and replace it. Removing door panels can be a little tricky to remove without damaging them if you have never removed one before but aren’t real difficult to work on.
I have an '03 Civic and it should be the same. Mine broke too several years ago.
There is one bolt that secures the whole assembly. Just below the “plunger” is a rubber cap over the screw that secures the assembly. Lift the cap and take out the screw, phillips head screwdriver needed. The assembly pulls out easily. I bought a new one but found the old one was OK is just needed to be snapped back in place. I took the new one back. Take yours off and see if it is truly broken.
You can buy a new one from Honda, not too expensive. Perhaps it is available cheaper on ebay.
Take out the bolt, the thingy comes out in your hand. Disconnect the old one, connect the new one. Put it back in the hole in the door jam and reattach the bolt. Replace rubber cap. 5 min. job. Easy DIY.