4".
You'll need the adapters from the front speakers, so don't throw it away. At the rear, the mounting holes aren't in a perfect circle. If you only have two thumbs and not ten, this is an easy job. Make sure to pull the speaker cables nicely under the carpet and plastic trims.
When you're at it, stretch a phono cable and power cable for future power amplifier.
Morten.
Abarth alloys. Inter Alloy lowering. Phoenix Gold & Kenwood audio. Abarth steering wheel. Insane Dalmatian door and seat refurnishing
Projects: K&N induction kit, racing seats, exhaust, window tinting.