Here's how the tester works. If when your normal game playfield wire harnesses are connected you're having switch issues, and you connect the 64 Switch Matrix Tester (along with its own wire harness) and also have issues with the same switches or all the switches in the same row.. then most likely you have a problem at the MPU board. With using completely different wire harnesses for the tester and still seeing the same results (ie. same switches are not registering when using the tester), you've eliminated playfield wiring harnesses and playfield switches as the problem.

Is it a chip on the board for that row/column? Is it a bad SIP resistor or some other component in the circuit? Is it a solder joint problem somewhere? To figure that out you'll need to trace the problem further at the MPU board. A logic probe can help with this.

TerryB wrote up a guide on how to use a logic probe here:
https://pinside.com/pinball/forum/topic/terrybs-guide-to-logic-probes

I also made a very basic tutorial years ago on logic probes @ http://techdose.com/electronics/Using-a-Logic-Probe/395/page1.html
I don't claim it'll help you tremendously compared to some other guides, but it might help give you a quick understanding.

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http://www.pinitech.com - "Pinball Inspired Technology"
NVRAM, Bally/Stern LED Displays & Mods for pinball machines

Manuals/Schematics for most if not all Williams games are available on the IPDB.org pages.

For High Speed:
https://www.ipdb.org/machine.cgi?gid=1176

Scroll down to "Documentation" and pick the English Manual.

CPU Layout & schematics starts at PDF page #56.
The switch matrix drives (IJ8 connector) are on PDF page #58 (top left).

Transistor for that column appears to be Q43 (2n3904). With a logic probe, check logic pulses at R73 to see if it's pulsing or stuck high/low. Can also check the output of Q43 by probing the IJ8 connector at pin #5 with the logic probe. Let's say the signal is pulsing at R73, but stuck high or low on the output of Q43... transistor is probably the culprit.

Just FYI sometimes those SIP resistors can go bad and cause issues, those you should be able to verify those with a multimeter (pin #1 is the common pin on those particular resistors, so you'd use a mutlimeter on ohms setting.. and compare pin #1 with the other pins).