The ornamental design for a base for a cartoy, as shown and described.
FIG. 1 is a front isometric view of a base for a car toy in accordance with the new design;
FIG. 2 is a front isometric view with additions thereof in broken line;
FIG. 3 is a front view thereof;
FIG. 4 is a back side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view.
The broken lines shown in FIGS. 1 and 2 are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.