If the tent you get is white, like the one in the picture, you can point a light up into it, and then the tent becomes a giant soft box. The light loses a lot of energy doing this, because the tent material isn’t designed  to reflect 100% of the light striking it, but the same thing happens to sunlight going in the other direction, and any sunlight coming through the material will diffuse and contribute to the total amount of light. A setup like this runs the risk of being too soft, pushing into flat territory, so you’d want to add a second light to use as your key, and use tent as fill.

If you don’t want to have to buy a second light, you could try rolling up one of the sides of the tent, and using ambient daylight (not direct sunlight) to add contrast to the image.  The amount of contrast will vary through the day, however, and will require you to roll up/down the sides of the tent, assuming the tent sits in direct sunlight all day.

For close-up shots of work being done to the car,  an LED panel would be easier to position, and get in the way less than a large soft box, plus it would double as a work light.  The closer a panel light is to its subject, the softer the light appears, which is why they’re good for closeups.