There is a 20 amp fuse blade type in line where I believe a fuseable link used to be.
Starter was acting as a generator and would blow the fuse.
When that fuse blows there is no power inside the car.
There originally is NO fuse in the starter circuit, and 20A is way too small to supply the interior. Just what is it we are working on here, year/ model? Maybe a photo of the fuse? So the wiriing is NOT factory?