Why do you want rear speakers?
If you're going to spend money on the audio, you'll get better sound by swapping in a headunit with decent amplification. Better yet, add a dedicated amp just powering the front speakers. One step further, add a sub.
If you go the amp route, you can get away with not even swapping the headunit.
In my old mk4 Gti, I kept the standard HU (afterall its just an input to the amp and is no longer doing any heavy lifting), sound deadened the interior, added 2x Genesis amplifiers, and a JL 10" sub. Amps and sub hidden under a false floor (with a custom made VW logo aluminium grill cover). The sound was pretty damn good in my opinion!
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