I LOVE car sales shops. I actually did start doing them as a newbie, but I can see why the advice here is that they are not best for newbies. They are definitely harder than a fast food assignment. You need good attention to detail, good notes/good memory, and really good writing skills. It helps to be car-knowledgeable.
I'm a car buff. It helped, too, that I did my first two with a family member who was already a mystery shopper. She went with me and pretended she wasn't interested, but later when she sat with me and I did the reports, she made suggestions and said things I had forgotten to look for. She read my report when I finished and said "What about this," "what about that?" She also made me re-write one of my narratives because she said my grammar was horrible. It was. Because she was a hard editor, I didn't get any editing questions when I submitted the reports. After those 2 shops, I went out on my own and I got a lot sharper. I've done a lot of car shops now and a lot of other kind of shops, too.
Try Monterey Mystery Shopping Co. They do car and motorcycle shops. I did my first two shops with them. They are my favorite.