When I try that I just get an "empty prototype" error on execution.

FYI:: The code Im using is as follows...
file_str_writeto(f, (char*)player.skill5, sizeof(CAR));

Where
var f = file_open_write(...);
typedef struct{...}CAR;
player.skill5 = sys_malloc(sizeof(CAR));


Sounds like this is something more than just a missing header entry.
Looks like I may need to wait for the next public release...