You should put all of your extern statements in to a header file that you #include in every C file that uses the variables, even the file where the variables get defined:
vars.h:
extern const uint8 BusyPin;
one.c:
#include "vars.h"
const uint8 BusyPin = 14
two.c:
#include "vars.h"
...
Had you done this, the compiler would have been able to detect the inconsistency in the declaration and the definition, and you would have gotten an error like this:
one.c:2: error 91: extern definition for 'BusyPin' mismatches with declaration.
vars.h:1: error 177: previously defined here
Also, your code will be more efficient if you use #defines instead of consts, because the consts actually get stored in flash and have to be read at run time.