In your server folder, go into dta. There is a file called scripts.pbo. You need to unpack it with the Pbo Manager. After unpacking it, go to 4_World\Entities\Vehicles, in there is a file called CarScript.c, open it and go to the function OnContact, just delete the complet function body and thats it. Repack the folder again as scripts.pbo and replace the old scripts.pbo with it. You should of course make a backup first.
Edit: Here is the modified version of the scripts.pbo if you have problems https://drive.google.com/file/d/15_uUE99yJrt_gO95L-jaTsXC6bmzg47K/view?usp=sharing
By "delete the complet function body" i meant
void OnContact( string zoneName, vector localPos, IEntity other, Contact data )
{
}
Now your car don't react at all if you drive agains something