Cave paintings are writings on various parts of a DeLorean, usually done at the factory or at the QAC (Quality Assurance Control) center. These conveyed information important to the manufacturing process, but often jokes were written, and doodles drawn, mostly for assembly line humor. When panels are taken off, these cave paintings are revealed.
The leading theory behind this cave painting is that it was done at a QAC center. Many of the CFR DeLoreans were cannibalized at QAC Centers, who likely saw how far gone Egull (VIN 559) was and wrote it as a joke.
Other than a few smudges on the right side, this door (the only original one) has been entirely untouched. Nothing written here!
This is most likely some factory jargon as this door has been previously replaced (as part of the aforementioned cannibalizing). Its meaning is unknown.
And underneath the upper trim panel is another cave painting:
A sideways "B". Meaning unknown.