Sorry for the late answer, GTAGarage does not send e-mails about comments. If you still need help, you can install the car with Gta Garage Mod Manager (a.k.a.: GGMM) When you open the Manager program you can select INSTALLER/ Manual Mod Installer. After that you must browse the downloaded txd and dff files and press install. The original dff and txd will be replaced with the downloaded ones (if you don't want to use a car forever, make a backup from the original car first)
Dude this car rocks!! it makes San Andreas look like Need For Speed Most Wanted, but better because it's a Rockstar Games car, it's awesome. I had a toy stock police car when I was a kid, it reminds me of this one