[header] How to buy an optional memory module for your car's navigation system [title] Determine the need for more memory for the car navigation system. [substeps] Consult the owner's manual for the car navigation system. Find the listed free memory space of the car navigation system. 
Note that the owner's manual of the car navigation system may list total memory space. The total memory space is not important to determine because the operating system of the car navigation system will occupy much of that space.