Good call on watching out for the yellowing, it’s a common issue with resin SLA prints. Although there are some really good resins out there, none of them will avoid this issue completely. You’re going to have to apply a UV-blocking clear coat over the finished print.

Some good resins to consider are Formlabs Clear Resin and Siraya Tech Sculpt Ultra Clear. For the UV-resistant coat, the easiest to use would be Krylon UV-Resistant Clear Acrylic Coating (in a spray can). If you have a spray gun, you could use an automotive-grade acrylic urethane clear coat like Lesonal Universal Clear.

Just bear in mind that these will not be lenses in the true sense of the word. They will not focus the light, only allow it to be transmitted. You cannot apply this same 3D printing approach to headlights, where the lens is critical to focus the light beam. Indicator lights and taillights you can probably get away with, but please make sure that the light travel and clarity are still safe on the road!

Unfortunately, the best resins to use for this application are casting resins. However, you could still use SLA to print the mold and then cast it in that. It’s an extra step, but I feel you’re going to get a more durable product that way.