As long as it doesn't start at all, it's not the WFS.
It sounds more like the MSTG isn't being supplied with power or isn't starting up because someone...
someone tampered with the PROMs, because the system light should also come on when the WFS is active.
Diagnosis can still be helpful. Either access is not possible, or...
(

A power supply, wiring, or software issue) or there are errors stored.
Best regards,
Rudi