These are not simply LEDs, these are modules with an internal circuit that provides teh necessary drive for the LEDs when used in a car's 12 V system (I presume).
It is not at all clear that it is possible to 'dim' these modules by applying a series resistor - it depends on the internal circuit of the modules.
Assuming that these are inexpensive modules, the internal circuit is probably only a mixture of series and parallel connected LEDs with some internal resistors for limiting the current. If so, it will be comparatively easy to dim the lights by adding a series resistor outside of the module.
You will have to know the voltage (Vdimmed) at the module's terminals at dimmed brightness, e.g. by connecting a module to a variable power supply (DC) and setting the voltage such that the dimmed brightness matches your expectations.
You'll also have to know the currrent (Idimmed) these modules draw at the reduced voltaeg (Vdimmed) supply. Measure this with the same setup as above.
From these values you can calculate the required series resistor:
Rseries = (12 V -Vdimmed)/Idimmed.
The power rating of this resistor needs to be:
Presistor = (12 V -Vdimmed)*Idimmed - for a good safety margin at at least 10 % on top of that calculated value.
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