you could take the bulb to a auto parts store and see if they have any available that are not as bright that fit the same socket. or could try these dimmers kind of expensive though
12 Volt DC Dimmer for LED, Halogen, Incandescent but its better to switch to led bulbs lots to choose from and plenty of dimmers available even remote control ones that are cheep can be removed from there case and mounted in the light that's what i did. those led dimmers will not work with regular bulbs though even though some say they can handle 8amps i burnt a couple up trying. some rv supply stores may still sell dimmers that work on regular bulbs but they have always been expensive. this is where i get my 12v led dimmers.
12v led dimmers