Some cali dude can correct me on the history of this. DJ Art Laboe coined the term "oldies" and put out the first oldies comps on his original sound label. These were pretty common, standard songs that became a part of the culture.
I think the original series of slightly rarer soul was the series "East Side Story", which actually came out on LPs. Here is a box set of all the albums:
https://www.cduniverse.com/search/xx/music/pid/3492840/a/East+Side+Story,+Vol.+1+-+12.htm
As you can see, the tracks aren't very rare. Then there was also a series (which I know wasn't totally licensed but supposedly semi-legit) that came out maybe 15 years ago (?) on a label called ITP called "Underground oldies". It did have some rarer 45s (I remember hearing the royal chessmen "begging you" the first time off these CDs, at the time I never understood how I managed to collect everything else so quickly but not the royal chessmen -- I do have a mint copy now though ). The company also licensed dore and renfro titles and put out out comps of these labels as well as including tracks from these labels on some of their CDs. They also put out material (which was mostly cover songs) by the LA Hispanic group "Sly, Slick, and Wicked" (the group that had 45s on Bad Boys).
The first set of rare, collector oriented comps (which were straight bootlegs) was called Lost Soul Oldies. These comps had lots of truly rare 45s and contributed a lot to collector oriented culture. It really got people into digging for records. They got distributed and even inspired collectors in other cities (I knew a bunch of chicago dudes that bought the whole series off the guys who put it out)... I have at least 5 titles in the series. They released a lot of titles.
Now there are a bazillion bootleg CD series that are out and it is part of the culture. There are clearly more fans than collectors so I'm guessing any of these CDs sells a lot of copies in California. Collectors are looking for rarer and rarer records trying to get stuff no one else has or knows about. Now a lot of stuff that's on the lost soul oldies seems basic and common because people are digging deeper and deeper. I guess at that point it's similar to the northern scene. It's a very collector oriented culture as people try to outdo each other with records and play their tracks on youtube, etc.
I think to get more of a history on the association with car shows, etc. you could look at Ruben Molina's book about lowrider music. But my point is that there is a collector culture independent of car shows, I think people are starting to do rare soul shows and obviously there are collectors in other cities. I think there's also some association with the same played out oldies and the car shows whereas younger collectors are looking for "rare" records.
It would be good if someone posted correcting any of this if it's not accurate.