Allow for the heater’s connection lead when positioning a standard mirror defogger.
The standard WarmlyYours mirror-defogger product guidance specifies a 36-inch (3 ft / 0.91 m) cold lead. This is the unheated electrical connection lead, not the size of the heating film.
Check the exact model and supplied lead before finalizing the mirror and junction-box positions, especially for a custom or older product. Follow the defogger installation manual and local electrical requirements. Do not cut the heating film to make the installation fit.
Sources: standard mirror-defogger product guidance and mirror-defogger installation manual.