I bought a pre-owned car. what is the blessing and procedure for a pre-owned car? I was told to buy a new top/shirt and say the blessing standing beside the car or sitting inside it wearing the new shirt.

Mazal Tov on your new car! You are correct that the purchase of a car warrants the blessing "shehecheyanu", as it certainly brings great pleasure to you. It does not matter that it is a used car, as the blessing relates to your happiness, and for you it is a "new" acquisition. So the blessing is recited on all important purchases that bring real joy to their owners, whether the item is new or used. So you don't need to buy any other new item to "cover" the car. The blessing may be recited in or out of the car. (And as this is during the Sfirat HaOmer period, I'll add that the shehecheyanu blessing may be recited even during this time of year). Blessings.