The Corvette is finally in GTA V, with full mods! Shame it doesn't replace the Coquette, but renaming the ytd, yft files and the individual files inside the "mods" file to Coquette's might work.
And when I do that it fundamentally breaks the YCA add-on pack. I've had to rely on the backup files for both that have nothing else but the YCA strings to get that particular pack to work again.
Try these steps.
1) Usually add-on packs need to be copied to both x64/dlcpacks and the update.rpf/dlc_patch path. Some packs only needs to be copied to the dlcpacks folder (which contains the dlc.rpf file) and thus only the dlclist will be affected
With YCA mods, you need to copy the dlc.rpf to dlcpacks folder under a folder named "ycamods" and the content.xml to dlc_patch folder with the same folder name of "ycamods". Once you've done that, the dlclist and extratitleupdatedata file must be added new lines like what ViscousTraction said. Then, I think its safer to check the vehicle.rpf inside the dlc.rpf file to check the name of the models to spawn them. Then just edit the "added cars" section in Simple Trainer's ini with the vehicles's name (zl1, lp700, gtrnismo) so that spawning is easier in-game.
2) If the game crashes, that might be because the order of the add-on files and the order of the add-on file lines in the dlclist and extratitle is not the same. (if you have more than one add-on packs, that is)
Sorry if its a bit hard to understand.
I've learned this the hard way, after countless crashes and remodding and all that hassle. The GTAV mods page should really put a section for newcomers on the basic steps of all kinds of vehicle and texture modding.