The inside headliner near the rear window and half way to side window needs to be unclipped and pulled down back, then unplug the wires and unscrew GPS receiver and old fin and replace with the new, plug wires back in and replace headliner. It's a job for an auto upholstery mechanic. This obviously refers to the sharkfin one. Some have another one for speed assessment on the inside of the windscreen I believe.