Although there are a number of drawing programs out there. I think Autocad is one of the best for designing car stickers. One of the obvious reasons is that Autocad is a vector program and vector files are required by plotters to cut the sticker.
A vector file is a computer drawing that has been produced by vector. A vector is an engineering term that means a line of force and direction. Because computers are mathematical machines tit is easy for them to create lines and shapes. A raster file is an image file that is made up of a series of dots and these type files such as Photoshop will not work in a plotter.
There are other reasons for using Autocad for your car sticker design it is simply easier to use. There is definitely a learning curve attached to it and it will be impossible to be adept at the whole program in a weekend, but a person probably could get good at some of the elementary commands in a short time.
For drawing car stickers,Autocad is better than some of the other vector drawing programs such as Coreldraw and Illustrator in a number of areas. One of those areas are basic drawing or sketching. It will be possible to duplicate most of what Autocad does in those other programs but it can be a lot more tedious. Autocad does a better job of understanding the artists intent so that when the person using it is doing some editing it is very easy. Autocad can be a lot more precise than the other programs as well, you can tell issue commands with coordinates instead of dragging a shape and then editing it in the other programs.
There is one area where the other programs are better than Autocad and that is anything involving text. Those programs are very creative when it comes to manipulating text. Another area they shine is with color. Autocad can do some very basic things with color but it can't do shadows or gradients and it does not have the palette available that Coreldraw or Illustrator does and certainly does not have the features such as creating a custom palette.
The great thing about Coreldraw is that it imports Autocad files into its drawings effortlessly and this makes for a new way to draw. If you are designing a car sticker that is going to have both text and graphics why not use both programs. Since Autocad is better at drawing lines and shapes, why not do that drawing in Autocad and then when you are done import the file into one of the other programs if you have to do any text or any special effects with color?