[lightedman]

"but monster wiring harnesses are not so cheap."

They aren't needed when the lighting is LED. The wiring harnesses going to more modern headlights are quite thin.

"I always imagined that newer cars would have a single CAN link to an ECU [0] in back, and that ECU would control all the lights near it."

They do but some are moving away because of the total lack of security and ability to compromise the CAN bus through the headlights to steal vehicles - read https://www.autoblog.com/news/vehicle-headlight-can-bus-inje... for what's going on there. They're too cheap to actually spend the money on real hardening so they're moving back to pure hardware control in many cases.