I would like to use RasPI with LibreELEC and KODI in my car, having a 16:9 screen with only composite input.
I've set up the Raspberry PI 3 Model B with current LibreELEC and KODI, and bought a composite cable.
But when using it, I see only a 4:3 "square" is filled on the screen, not the whole 16:9 screen. Black areas are remaining on left and right side.
I have the same 4:3 result, if I attach it to my 16:9 flatscreen television at home via composite. (HDMI works fine there in 16:9, but I don't have HDMI in the car.)
I tried to move the corners in the KODI video settings to adapt the picture. This works, to set the picture size exactly to the visible 4:3 field. But if I move the corners into the black areas at both sides, the corners disappear behind the black. 
I've read instructions for other OS, to set in config.txt:
sdtv_aspect=3 # 16:9
sdtv_mode=2 # PAL for Europe
I've tried this configuration values in LibreELEC, but unfortunately it did not change anything, the issue is still the same.
What can I do, to use the full 16:9 screen using composite video signal? Thanks for any hint!