The tailight has undergone a metamorphosis in the last couple of years. From Henry Ford until the new millenium, the humble tailight hadn’t changed. It was very simple, all that was involved was a battery.. a wire to the tailight switch, a wire to the tailight, and then a wire back to the battery. Easy to understand by any human, and easy to diagnose. All you needed was a $20 test light and possibly (but not necessarily) a schematic.
Come the new millenium and the age of the computer and things are different. Tailights (the kind that still use a bulb, that is) now consist of a battery, a wire to the switch, a wire to the bulb, a wire to a computer, and a wire back to the battery. They now work by pwm (Pulse Width Modulation), which means you feed a bulb with power from the battery, and the ground side is pulsed by the computer. The frequency of the pulse determines how bright the bulb is. This does have several advantages, like the brake lites and tailights can all be the same bulb (simplifying logistics.) With the computer involved, you can also have any bulb take over any other bulb’s function. (like if a brake lite bulb fails, the tailight bulb can take over its function.) Diagnosis is more difficult, however. Now you can’t use a testlight because it’s impedence can blow the computer. Now you need $500 Digital Multimeters and a $4000 Scan tool to talk to the computer.
The net effect is that to install tailights in your car, it used to cost the manufacturer about 20 Bucks. Now it costs 500 bucks..
I think they call it progress…