The ‘Detailing’ Slot would come after Dry Finish and before Polishing.
Detailing would allow the addition of decals, custom paint work, logos etc.
Detailing could be a research item under the Paint specialisation.
Detailing work could be applied to any current vehicle as Polishing is now.
(There would be low demand for custom cars but I see this as a mechanic to add variety late game)
Limited use with the current game processes but should the idea of ‘Contracts’ be added then the Detailing slot could be used for example to add Taxi livery to (yellow only? choose colour) cars or Sign-written ‘panels’ added to small Vans for a fleet contract of maintenance vehicles.
Each Detailing specialisation (Taxi livery/ Fleet Maintenance / Police Vehicles etc ) is researched as a subcategory of the Detailing specialisation.