Hi I currently have a 60 plate fiesta zetec s. I am looking at upgrading the sound system to what's in the new ST The Sony headunit. Does anyone know if this would also require a whole speaker upgrade or age the speakers the same as what's in the zetec s?

HU will fit right in, you'll need the wiring loom that comes with it though, and I mean the whole thing as it's completely different, speakers will be fine, same power output iirc, although it's been said by many people the standard stereo is actually a better quality unit sound wise, more bassy although no DAB.

I can recommend a speaker upgrade. The factory fitted Sony headunit comes with premium speakers. The standard speakers are made of some kind of paper, so the quality is not that good.

Part numbers of the premium speakers:

Front: AA6T-18808-FA
Rear: AA6T-18808-GA

Be aware that the MK7 facelift (ST) Sony Headunit is different from the MK7 sony headunit. I'm not sure if it is compatible, but to be sure you got to search for a pre-facelift Sony headunit.

You will lose Aux, usb and Bluetooth going from a pre-facelift car/unit to facelift car/unit due to the latter hang SYNC. Been there done that :(

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A speaker upgrade would be something I'd look into doing soon. Has anyone got the specs of the "premium" speakers over the one speakers that come with the standard headunit? My ZS has the six speakers, but, as it doesn't have a Sony headunit, I assume the speakers are otherwise standard?

The output power of both speakers are the same. 4 ohms and 20/25 Watt.
The build quality of the premium speakers is much better, so the sound quality is better as well.

No Sony headunit = No premium speakers (factory fitted)

If you wan't to improve your sound quality you can also consider placing b-max tweeters in the back.
There is little room for error, because there is very little space. I did place them, and the sound is more 'spacious' (not sure if that is the right English word :P ).