Low beam lights flicker

I have a 2007 GT with led bulbs installed(tried 2 different pairs) and when I play my stereo loud the lights will flicker very fast. They don't dim with the bass line like they would if the alternator wasn't powerful enough. I have a digital voltmeter installed in the dash and there is no voltage drop whatsoever when this happens or any other time. I'm thinking it has something to do with how the lighting circuit is wired in these cars but don't know for sure. Can anyone shed some insight into this weird problem?

I'm running LED bulbs in my 2010 GT as well and experiencing some flickering, though it's a bit unusual. Instead of a rapid strobing, my (especially right) LED slightly loses intensity for about 10 minutes at a time before returning to full brightness.

I've been considering swapping the entire assembly for LED halo headlights, mostly for the aesthetics, but I'm following this thread to see if there's a simpler fix first.

I have searched for anything related to this problem for awhile and have turned up absolutely nothing. The way the Mustang headlight circuit is wired is strange to me and most likely the cause of the problem. The low beams get power from the SJB but I can't find any info on how the SJB functions. I suspect that my stereo is somehow effecting the SJBs power supply which causes my lights to flicker for some reason. I am thinking about wiring the headlights through 2 relays and using the low beam wire to trigger the relays. I have these little relay boards from Amazon that have a high or low trigger circuit with a capacitor on the board. My hope would be that the capacitor has enough power to keep the relay contacts closed even while the relay coil power supply isn't stable.

I suspect the LEDs don’t draw enough current to complete the circuit properly.
Did you try a halogen lamp? Does that flicker?

Wayyy back I put LED headlights in my car and they caused issues with, of all things, the TPMS. Pulled them out and went back to regular bulbs never had an issue again.

I haven't tried halogen bulbs because they will almost certainly dim when I have my stereo cranked up loud. I had halogens in my truck with a 250 amp alternator, 0awg wiring, and big 3 upgrade and they still dimmed. My tpms sensors are long gone and I disabled them using forscan awhile back so the annoying message never popped up so I wouldn't know if the leds cause issues with that. I did wire the low beams up using 2 relays but haven't had the chance to test them yet because I haven't left my house in the dark yet.

If the leds didn't have a high enough resistor in them wouldn't the car tell me I have a bulb issue? I can read dtcs set in modules that don't set the light on the dash and I do have a code for my trunk light and tail lights but nothing for headlights. The tails are sequential LED styled like the 2011+ cars. I really wish I could find some information on what exactly goes on inside the SJB. It seems to be a mystery box that is the source of many problems.

Just wanted to report that the relays have fixed the flicker problem. My suspicion about the power to the SJB seems to be spot on.