not sure....but think if its a well known dogging car park....then its a good chance they know and are there for that reason. maybe park a little closer next time...get out car..peek over...if they don't say anything..maybe just slowly edge a little more forward.
Certainly would approach car especially if you are in a car park known for dogging but certainly wouldnt approach car with your cock out thats asking for trouble. Slowly approach and make sure they see you then they will either stop or carry on or tell you where to go. If they carry on just watch from side of car and if they open window or door take it from there but dont forget some couples only want to be watched and not any involvement with others so be happy with whatever they want.