I thought it was one of Jimmy shines projects,could be wrong ...yeah I paint and modify them not just for mustangs but for other makes and models as well.
I really don't think you have any holes ,you could possibly remove your inner back side panel and and remove your back seat to check .that would be easy .
There is a company called gen5diy , he makes conversion harnesses for camaros and other vehicles as well for headlight and bumper conversions ,this would be something definitely up there alley.