Can't see any lights of other player on server

Hello everyone!

I have the following problem: When my friend (on our private server) turns on any of his lights (headlights, turning signals, reverse lights, brake lights, etc.) I can’t see them. I have tried it on a public server, and could see the lights of everyone, except for him.
Edit: I can, however, see his police light bar.

If anybody has an idea of how I can see his lights, please share the idea here!
Thanks