if anyone could tell me the factory size, that'd be awesome.

 

6.5" fit into those doors.

 

Or a oversize 6.75" as well.

 

Factory though were 5 1/4 right? Or no? 6.5" will fit with some cutting and watch the depth to the window mechanism behind.

 

They take Ford's retarded 6" driver that they call a 5.5" I think. A 'standard' Pioneer, Kenwood, Sony, etc 6.5" fits the hole.

 

1990-91 Ford Mustang Stereo Wiring
Constant 12V+ Green/Yellow
Switched 12V+ Yellow/Black
Ground Chassis
Illumination Blue/Red
Dimmer n/a
Antenna Trigger Orange/Blue
Antenna Right Front
Front Speakers 5 1/4" Doors
Left Front (+) Orange/Green
Left Front (-) Light Blue/White
Right Front (+) White/Green
Right Front (-) Green/Orange
Rear Speakers 5" x 7" Side Panels
Left Rear (+) Pink/Green
Left Rear (-) Pink/Blue
Right Rear (+) Pink/Blue
Right Rear (-) Green/Orange
Hope that helps you.
Nick