This problem and fix occurs on 2015 Mustangs and newer. Symptoms include, loss of power when driving, unable to set speed control, green light staying lit on start button after car is off, brake lights remaining on when car is off. This is due to the car thinking the brake pedal is not fully depressed. Replacing the broken rubber peice will fix this issue.
Broken rubber brake pedal stop pad, often times found on the carpet near the pedals.
Rubber feet I had laying around that I used to fix the issue, with adhesive on bottom.
Before the repair.
After the repair.
Email me at firstname.lastname@example.org with any questions!
Here's a possible more permanent fix: