Two notes:

First, you'd use "thou" here, as it's the nominative case, and the correct conjugation would be "canst".

Secondly, "thee" and "thou" were very much the form used to speak to a close friend or social inferior in early modern English. "You" functioned more like French "vous" or German "Sie"- as either a plural formation or a formal address of a social superior.

With that in mind, it would probably be more accurate to say that they should be rendered "Marcus, can you get the car, please?" and "Mrs. Miller, canst thou get the car, please?" And notice the parallel between "canst thou" and "kannst Du"!