I suspect they are sealed units. The rubber ring holds the whole light assembly in, you pop the light out, and put in a whole new light assembly, the bulb is sealed inside. (I suggest replacing with LED)
You generally will need to find a replacement of the same diameter, and hopefully the same plug (it you'll have to re-wire it). You can find them on Amazon and specialty sites like
https://www.truck-lite.com/, and probably some RV Supply websites.
The sizes and plugs are pretty universal so there is a good chance you can find a direct drop in replacement.