LED replacement bulbs

LED bulb runs on about 0.5 to 0.75v

To run in a car it either hasn't to have some sort of voltage regulator or resister to lower the voltage. Or the 0.5v bulbs have to be wired in series internally to add up to about 12V so 24 LEDs .

All that being said...

If it uses a voltage regulator it will generate some heat but much less than an incondescent bulb

If it uses a resister it will generate as much heat as an incondescent AND draw the same current as the incondescent. Totally defeats the purpose.

If it has about 24 internal LEDs it would run about as hot as a voltage regulated version.