I've told you how to put the new car in the \TRUCKS directory for use with NR1999/TPTCC99. Now let's heat up the engine!
Get back into NR1999. On the main menu, select "Driver Info". First, click on "Vehicle Type:" on the top-right and select "Trucks". You see two lists of all your fellow competitors. The list on the left are the non-active drivers. They're just sitting in the garage area waiting for their turn to run. The drivers on the right-side list, are the active ones.
Your new car is somewhere on the left-sided list. Select it by clicking on the driver's name. Next, click on the big right-pointed triangle in the middle to move the car to the current list. It will be added to be bottom of the list.
If you want to drive the new car yourself, click on the driver name on the right-side list. Press the "<" key as often as needed to move the name all the way to the top. As you know, the top-most listed car is the one you drive. Click on "Save" to save the modified list and on "Return" to get back to the main menu.
You're now ready to drive your new car! Enjoy! And mind you, don't come to me complaining about the car if you crashed it on your first lap. I don't work the pedals, you know! ;->