Instrument flashers out

I’ve been looking high and low to find out why my instrument flashers aren’t lighting. Both don’t light even though I hear the clicking and all exterior blinkers work fine. No electrical issues with this car, nothing altered from stock wiring. Where is the power source or a ground I need to check that these are tied to so I can start finding the cause. Maybe a fuse? I have the shop manual but still can’t lock down what the root cause is. Any thoughts on where to start? The instrument panel has been removed as I am restoring the heater box at the moment.