They look like the PL-9731P. The input voltage spec is 10-30vdc. That implies that they maintain the designed brightness over a range of input voltages. Below 10v, they will probably shut off. Above 30v, they might overheat and shut off.
Most LEDs can’t be dimmed. Ususlly, the ciruitry inside will give a constant amount of power to the LED emitters over a wide range of input voltages. To dim the LEDs, you need a circuit within the housing that is designed for dimming.