But that's okay. Once the individual is in the back of the car, they can, as the Americans say, be Mirandized. I don't think it's all that difficult a situation. I don't think the police officer has to get into a fight with a suspect and read him his rights at the same time. He clearly can do that afterwards.
A problem might occur if the accused were to make some kind of spontaneous declaration. Then you might get into a complicated situation.