Right on the money LeonardA, you are absolutely correct
The most probable source of this squeak is from the upper control arm bushings which are of a metal on metal design on your Mustang.
Because of this setup, the bushings need to be lubricated faithfully, penetrating oil won't help at all.
Some so-called 'mechanics' used to burn holes with a torch in the inner fender to gain access to the grease fittings, a true sign of a butcher
Google upper control arm bushing lube kit and your good to go