It is difficult to sew on a backpack which has a dense lining. Unrip a backpack, and then re-sew, too, is not the best way to go. Therefore, the first stitch made on normal fabric, then sewn to the backpack "secret" stitch. This allows you to personalize your bag without breaking its appearance.
Here are other ways to sew a Cars machine embroidery design or logo on the backpack.
Here is a small tips: Need to hoop tear-away stabilizer and you pin the backpack to the top of the stabilizer. One way you could do it is to make a patch with the machine embroidery design design in a coordinating material and then just sew the patch where you want it. If you have the Brother Pr600 (PR700, PR800, PR1000, may be another embroidery machines with same opportunities) embroidery machine and the fast frames therefore you can hoop the backpack. And you can hoop stabilizer and you pin the backpack to the top of the stabilizer, just hoop the stabilizer and then pin the backpack to the stabilizer. You would be sure and stay with it while it sews in case it shifts, etc.
You hoop the embroidery stabilizer (only for embroidery machines) and use a spray adhesive (spray the stabilizer, not the backpack) and also pin it securely to the tear away stabilizer. You need to turn it inside out to keep the excess material out of your way while you embroider.
Warning: it depends on the style of the backpack. You could use a water soluble stabilizer on top, although I'm not sure if it is necessary.