Sage advice from Milcom/CamO. Also look to see if flasher solenoid is going bad.
Cheapest thing is to check for bad lightbulbs.
If money is an issue always disconnect the battery before electrical work, and then check all contacts, check bulbs, then flasher control solenoid, and finally the overpriced switch.
Remote repair is one of those times that both parties need to be on the same page or it turn into a training session... AND that's OK, it's what we do here.
It gets a bit sideways when the problem is compounded with "I got this and that, and one of these too going on" Trouble shooting skills come out (along with the prints / drawings) and start the process of elimination and reconstruction back to standard, maybe even better that new.
While my responses may not be the 'easy fix" one is looking for, sorry. It is very difficult to diagnosis problems stated in a simple paragraph or two. Hence the sometimes long road down a rabbit hole or two with these threads, here we go. Yes, it is a fun ride. BUT for this old guy... I try to point to an obvious, check this or that. Like in check switch, or bulb or is in plugged-in? LOL with you...NOT at anyone.
So...while "lights out" may seem simple, there is a lot of points of failure along the way. (over voltage regulator YEP, that too)
In the end it will be a simple thing or two or compounded by a series of events that need to be reconstructed. (sounds like doctor talk)
Focused guessing is all I got while poking and prodding the problem based on provided info. And if the gang here hasn't notice, I poke a little bit at the OP to step up, dig in. Your are the "boots on the ground" so report out with good data. Pictures can speak a 1000 words. Just like an X-Ray.
HMMWV is a total all hands on deck machine IMO. All of the responses here are the collective brain trust of years of HMMWV and researching the TM's .
Start with the bulb/s, check for good and work it toward the switch, flasher and anything wire, connector, socket, loosey, goosey, rusted, busted and I am all out of guess.
GOOD LUCK...and is there a doctor in the house?
Don't be looking for the "flasher solenoid". I'am guessing that's not it, CAMO
Free information and observations. Usually not directed at anyone person, anytime, anywhere or any HMMWV.
Just working the keyboard, burning bandwidth and watching TV.