Ok, so you want to check the wire from A3J3 pin 10 and make sure you have continuity from the connector through ALL of the switches. The wire colour code is 411 which is a white wire with a yellow and 2 brown stripes. Use your DMM in continuity test and ring out from the a3J3 connector pin 10 to EACH SWITCH (with the connector disconnected from the PCB) to make sure there are no breaks, and visually inspect each joint and look for any kinks in the wire that may cause intermittent issues. If that all checks out, then test the diode(s) associated with pin 10 of the A3J3 connector using your DMM (this is the A17P1 connector on the diode board), and then also test the 220 ohm resistor tied to that row on the same board. (these have been known to burn up / go open)
Now if all that checks out, this is going to get fun. Sys3 switch matrix shares it's strobes lines with the lamp matrix. (see back to my "fun" comment) What this means is, if everything in the switch matrix is checking out, the source of the issue could be in the lamp matrix. You'll have to check the lamp matrix chart and locate what row of lamps also uses the 'strobe 1' line, and check all of these sockets for a short. Could be a bad bulb, a stray wire but most likely a bad socket coated in organic flux. The factory used solder with organic flux and this tends to be conductive, so you'll have to check for that and there could be flux on a socket that is causing the issue. If you notice any sockets that look at all suspect, try cleaning any / all flux off (with 99% iso, or flux cleaner is best if you have some) and eliminate any shorts.
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