Siri Eyes Free not working on iPhone 6s Plus but fine with iPhone 5

I have a Pioneer car headunit which support Siri Eyes Free. This works fine with my wife's iPhone 5; showing 'I'm Listening' on the car head unit display and providing full voice feedback etc.

However, with my iPhone 6s Plus, when I try and trigger Siri Eyes Free I just get the normal Siri via my phone, the phone screen lights etc and provides visual feedback only on the phone. The car head unit just shows 'voice control'. Audio is still routed via the car head unit however.

How can I get Siri Eyes Free working properly on my iPhone 6s Plus?

Many thanks!