You have to be careful when editing those .dat files, if you mess up a single dot, number or whatever your game will crash.
Make sure after editing those files that they're still .dat files (and not .txt files)
I recommend using simple native trainer to check if those cars work
Or did you mean you can't install them at all? Are you using sparkIV/OpenIV to place them into the vehicles.img? Alot of modders give their vehicle files a custom name, if you import those in SparkIV it won't recognize them indeed, you first need to name them (give them the same name as the vehicle you want to replace).
In OpenIV you can simply right-click the vehicle you want to replace with it, hit 'replace' and select the custom car file