Could just be the voltage drop through the wiring loom to the fag lighter. Run a nice chunky wire direct from a good 12V supply with a good in line fuse holder. Cigarette lighters, phone chargers and sat navs wil have a nice wide operating voltage of between 10 and 14 Volts, a high speed digital interface like HDMI will rely on a nice clean source voltage with no noise or inteference.
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