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Hello and Welcome to justanswer..My name is MIKE..You don't mention the year of your Prius - but all Toyota passenger vehicles since 2006 have the ISOFIX system installed..To access the isofix points you need to raise the rear seat cushion - there you will see the anchor fixing points ( Isofix system ) for the child / baby seats.This image should help you further. This is an actual image of a Prius seat - Isofix system.
Best Rgds, MIKE.Let me know if you need anything more on this at all - Please do click on the OK rating to Excellent ratings service for me now, Thank you for that... and you can get back to me anytime if you need / wish.. Best Rgds, MIKE. Please note, you are rating the content of my "answer" - my help to you - not the "system" which can experience delays etc which are not under my personal control -or delays caused by "time -off" which we all have on occasion - Thank you.