The DVD player came with my truck’s movie system. The DVD player has RCA plugs on it (Video, Audio (R+L)) inputs. How can I play movies from the external hard drive through this system in my vehicle?
Your DVD player can't be connected directly to the hard drive (the hard drive is a generic digital storage device, and DVD will only take analogue inputs). One workaround would be to connect the hard drive to your computer and produce DVDs which you could play in your truck.
There may be a small streaming device with analogue output which will work for you, but I'll leave this for others with more knowlege to suggest.