PWM itself is fine!!! I have no idea why they set the PWM frequency so low. Even setting it to something like incredibly low like 1kHz would have solved the problem. In electronics land you almost have to try to get frequencies that low out of e.g. a microcontroller - you need a very high clock divide ratio to get a timer PWM period that long.

I think they have actually done so, because I am noticing fewer low-frequency taillights these days.

From vague memory of reading some data sheets: there are cheap little constant current LED driver modules for automotive applications. Two wires in: PWM power. Two wires out: the LED array. To get anything less than full output, you need to drive it with a square wave, and it might not function at a civilized frequency of a few kHz.

Doing better would require a different wiring design — there’s no way to just swap the driver without making the driver fancy enough to take, say, 50% PWM in and produce half current DC out. (Obviously this is trivial, and even available entirely off the shelf for non-automotive applications, if you have three wires in. But you don’t.)