The vehicle's autopilot rolls 12 dice for maneuvering, but only 5 dice (default) for stealth, same stealth as Mercer. I'm not sure Grid Guide has a "follow that car option", and it certainly doesn't have a "Follow that car stealthily" option, but I'm hoping that a good Pilot program can operate independently of grid guide. If Mercer has to do the driving, he only has 3 dice (defaulting from a REA of 4). Let me know how you want to proceed and how many of each you want.
Also, I'm planning on swapping out his wood allergy. It seemed neat at the time, but he's got wood knucks, etc, and it would just get ridiculous trying to keep track of his wood exposure when he's practically always exposed to it. Just wanted to bring that up before it became relevant. Since I have 5 unspent karma, I might just go for a 5 point NQ and use the karma to make up the difference.