How to remove flickering on car-powered LEDs?
A high-quality car capacitor should work.  The bigger it is the smoother the signal.  Bigger capacitors do take longer to charge and cost more. 

Amazon has these.

Just be careful it does not short, there is no overvoltage, it does not get connected backward, and it can handle your current load.  If something is wired wrong, these capacitors may explode.  Just make sure the top of the capacitor is not pointed to anything important and any escaping gasses can vent somewhere safe in the event of an explosion.  

A fuse or better yet breaker which automatically resets would be useful to prevent a short from damaging setup. 

A switch which can disconnect the capacitor when the engine is not running may save your battery life.