Hope this helps someone...
Son bought a 2002 Honda civic ex off Facebook marketplace. Ran for a few months and started having electrical issues. Called one night when it died, sent a video of the dash lighting up and then going out. Started back up, got it home and then died again.
Started troubleshooting, Symptom 1, battery when not connected reads 12v, when cables connected, 3+ volts. Tested all fuses both engine compartment and under dash. Using off the Internet guides, got good reading back to ignition switch connector, based on troubleshooting guides replaced ignition switch.
Then, no power to anything, no lights, no dashboard, no nothing. Hazzard switch light clicked a few times faintly then died. Battery is good, all fuses appear to be good. Ordered official Honda Service Tech books ($145) and three weeks to shipping. Went through all diagnostics that pointed to the MCU under the dash.
Bought a new MCU from Honda ($450) and attempted to plug in. The confusing part of the MCU is there are several of the same connectors that fit both the front and the back. Still no power to anything and I mean no power to lights, external or internal, no dash, no crank, no nothing so the whole car is dead.

Car was towed to a different spot. Then towed to a different spot. Battery still at 12v. Connected the battery and since there was a car in front, noticed the passenger side running light was on as well as the drivers side headlight. Turned the key and now had dash lights, but the windshield wipers were on constantly and could not turn them off. Again tested all of the fuses with no indication of a problem. Pulled the headlight assemblies (non OEM) and discovered a hacked up job of scraping main wires and wrapping new wires in with electrical tape. Fixed those with proper connectors. Lights are still on constantly. Studied the manual for about an hour and traced out the connectors to the MCU and discovered that duh, all of the dashboard connectors are in a similar bundle and they all connect to the rear of the MCU. All of the engine compartment and auxiliaries connect to the front.

Hook the battery up, and now, no power to anything again, however, the brake lights are on and won't turn off. Connected battery now shows 12v. Car came with a new alternator that was installed by son, incorrectly, I transferred the tensioner bolt that was missing but did not hook up the alternator wires. Accidently hit the power wire to the ground which sparked. Upon connecting, found one of the wires had pulled out of their connectors, fixed that.

Short story long, I believe the 80 amp fuse was going, but still had enough of a connection to intermittently supply power to random things but not enough to power everything. After sparking the alternator it fully blew the 80 amp fuse that tested good previously as well as two (2) 15 amp fuses for the coil and something else in the under dash mcu. After replacing the 80 amp fuse, car has full power, cranks and all electrical systems are good.