Measure D3 to ground on each chip again and write down the resistance measurement. Most times the one that measures .2 or .3 ohms when all others are .5 to 2 ohms likely is the issue.

This is where testing resistance over just using the continuity beep comes in handy. Continuity test will beep even at 100 ohms. Easy to overlook a bad fuse that should measure less than 2 ohms. Yes, I have seen a fuse before measure about 100 ohms. This is why i have been looking at the resistance measurrement on fuses vs using the continuity test for over 10 years now.

Just my experience. This troubleshooting trick may not work well with multiple chips sharing the same data line.

What kind of funky stuff?

I had worked on a driver board in the past where a data line was shorted like this. It appears to been caused by someone shorting an insert lamp to a flasher or a coil while power was on.

Failure: One whole column of insert lamps, some coils and flashers were dead.

This likely should only isolate to U1 to U5 buffer chips.

If I remember from 2 years ago on this board, this is how I troubleshooted it. Once I found the shorted data line, I had diode tested all the pins on U1 to U5 and came across U1 having 3 pins measuring .1 to .3 volts on the diode test. A couple of these same pins were also common to either U2 or U3. Since U1 had the higher number of pins with what amounts to pins shorted internally, I replaced U1 and it fixed this board.

When troubleshooting an issue like this, best to measure the board again once the part is removed in case that was not the issue or you have more than one shorted part.