Car Ground Point Requirements
Carbon does conduct well, but carbon fibre panels are a whole lot of carbon strands mixed in with epoxy which is a good insulator. It might work, but you may only have a handful of carbon threads doing the actual conducting and it will be totally inconsistent and prone to failure. Aluminium on the other hand will perform better than steel as it has less resistance.

However, you shouldn't use the chassis for current return paths. If you look at a modern vehicle wiring loom, you'll see they run separate ground wires for just about everything. At some point (normally near the ECU) they will affix a single ground to the body shell but this is actually just to use the body as a shield rather than a current return connection.

The reason for this is that (at low frequencies anyway) loops are what generate EM noise and receive EM interference. If you are using the car body as the return path you are creating a bunch of large and inconsistent loops which will pick up noise very well. Even if your own car is okay, park next to a guy with solid copper plug leads and watch all your gadgets go crazy.

You can share ground wires between common things (such as light clusters) but if in doubt give it a separate wire right back to a common ground point.