Based upon all of the chips socketed under the battery pack I'd say this board had heavy battery "pollution" that was cleaned and repopulated.

You should be able to compare the logic signals at the PIA with the pins on the display header. They go through several SRCs but they don't alter the signal. If you get another signal at the hear then the srcs affected may need to be replaced

It can be a bad display too. Can you test this in taxi? You should be able to find the replacement SRCs at great plains.

Don't replace them until you know they are bad. The logic probe should give you the same value on the header pin that it does at the pia pin that drives that signal. If it doesn't then check the signal at the src that is in between them.

The SRC is just a set of resistors. Use a DMM to test the resistance. Black on the common ground and red on each pin.

Are you running a test rom for your tests? That would be the only way to compare apples to apples here