That seems to have been a propaganda act, as best as I have been able to pin down it was short-lived, she never became a (real) mechanic and she slept in the Royal residence at night.
The entire WWII experience as an active (on her 16th birthday she was appointed Colonel of the Grenadier Guards during WWII.The Grenadier Guards training element was stationed at her home, Windsor Castle as security and personal escorts for her.) military member for Princess Elizabeth seems to have consisted of about 6 or 7 weeks when she lived in Windsor Castle, enlisted as a Lieutenant, attended a personalized 3 week course, had some photos taken of her touching trucks, never left the house, was promoted to Captain and was finished with the entire charade by the 6th or 7th week.
Btw, it IS where she learned to drive.