Originally Posted by
kiddion
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There is no workaround, however anonymous vehicles can be sold, just not the high end (like all regular vehicles in the game). For cars that are spawn as personal vehicle, I would expect you could mod them at least in your own garage...
The list of vehicle upgrades are almost 100% unique for every vehicle model. And every model has its own set of mods and identifiers. And if you use an invalid value the game will crash...
Since I don't have a list for every vehicle model, upgrading manually isn't possible for now. But every anonymous/personal vehicle should come with performance upgrades.