I wanted to play movies that I have on my HDD through the rear entertainment system in my minivan. The minivan has a 110V outlet and composite "IN" in the back left and it has a HDMI port in the back right. Can I just get a cable that has one end going from my HDD and the other end into HDMI? Will that work? Any help is appreciated.
As said you need something to take the video file from the HDD and convert it to the video to be displayed. A Roku3 should work well for you. Also if the DVD player is something you can upgrade, a new unit might had a usb input port that would support video formats.
HDD is a DUMB storage device, it doesn't know what to do until SOMEBODY tells it to. Both composite/HDMI hookups require some sort of "outboard intelligence" like a laptop between the HD and the display unit.
I recommend upgrade the VAN system with something with a USB port that you can attach, and can "talk" to a USB-attached HD. Note video format supported, more the better obviously.