Using memory editor and not tuning shop
In this example is used Cheat Engine 6.1
Once you have located car in memory (followed the car hack)
Now it is needed to click on "memory view" in Cheat Engine
Right click on the memory area and select "go to address"
Enter the car address (address that changes car, explained in car hack)
Now from there, 5th row below car address, is the tuning slots
See this image for details