Originally Posted by wpoll

It IS set up that way. The LKM uses pulse-width modulation to vary the effective voltage across the lamp (the lamps filament acting as a averaging mechanism).

Pre-facelift cars have dual filaments and no PWM. Facelift cars use single filament lamps and PWM to alter the brightness.

This is why LED tails are both hard to source and often don't work well anyway on facelift (LCI) cars.