To the left of the pits section at the Autorcoss arena, right after you leave the pits
There's one primary way to get there that was discovered back in previous patches, but unfortunately, the new "RETIRED" thing comes up when you try to drive to it. I've created a start point that goes around the retire system but you have to drive down an exact path or else it kicks in and resets you.. I'll try to upload it for you some time