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Didelphis virginiana
It's an opossum. I don't know what species lives in Southern California. This one was the size of a large house cat. It was at the bottom of a 4' deep concrete trench, laying on its side and attracting flies. We got it to lick water off a branch. It perked up a bit, but we scooped it into a box and took it to project wildlife for a checkup anyway.
This was a concrete trench on a college campus...probably not choice habitat.
There are no other native species of marsupials except for Didelphis virginiana. They weren't originally found in the western US but have expanded their range either through the pet trade or introduced otherwise. They are very adaptable and do fairly well in suburban areas.
I would agree with J's suggestion. This is a Virginia Opossum.
Please confirm species ID. I know Didelphis virginiana inhabits San Diego, just a matter of if there are neighboring species...