My guess, based on your symptoms: Something in the left-side circuit is open.
The gauge light circuit is completed through the lamps on the opposite side (to the side that's flashing). So if your right side flashes, and your left side is open somewhere, the idiot light won't flash. The situation for your left side is just the opposite.
After checking the bulbs and sockets just to rule them out, I'd check all connectors in the left-side circuit, probably easier said than done. You'll need a wiring diagram. Since it's happening to both left lights, the break in the circuit is likely in the wiring that's common to both of them.
Thanks Marty. I did a cursory look at the wiring and fuses today. I installed the super mini lights (not LED) so it probably was my wiring job at the rear harness. Looking at those lights I don't think they're replaceable--certainly doesn't look like it, anyway.
Good info on how the gauge ckt is completed, thanks again.
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