rear deck speakers shaker 500

I searched around and didn't see this in a thread.

'11 GT with shaker 500

I noticed the cheap rear deck speakers seem to attached to the deck with no internal ( cabin) openings . The deck doesn't seem to have any evidence of a port/grill cloth , just a solid piece .. seems odd..

SO is Ford relying on the resonance of the decklid ? or am I missing something?

I plan on upgrading , just curious as to whether ported rear deck speaker openings is a normal mod

Thanks in advance

I crawled back there over the weekend . sound comes out fine however it's rigged .. I'm sure a decent set of speakers will be an improvement and the access is easy .