There are 5 cars in Thief Simulator, the hardest of which to steal is the Kubra GTS which is located in house 207. The only way on to the property is through the front gate that you need to use the hacking PDA on. I recommend starting around 9 PM. Once you get through the gate hug the left hand side, climbing vines and dodging security camera until you find the pool. The lady that owns the house spends a lot of time here and you should be able to copy her key signal easily. on the left hand side of the building you should be able to see a climbable rain gutter from the poolside. take that and enter the second floor. There is a security camera that you can either disable at the terminal at the intersection at the top of the left hand side of the stairs, or simply try dodging it. at the bottom of the stairs take a right and you're in the garage. I would recommend staying here until 3 AM when Mrs. Foster returns home and the guards change position before you use the gate control next to the garage door opener and drive your way to freedom.
I have also made a video guide showing how I stole each car if you're having a hard time.