Mine _does_ work below the spoiler. But most of the time that part is too smudgy to really see the difference! Is that your case too?
I also don't see how Ombrello could keep that part substantially more clean than anything else (including no treatment). Dirt is simply 'sucked' to that area because of the airflow around the spoiler and back of the car. Nothing will stop that dirt from landing there. It might come of easier with rain, but....it never rains below the spoiler?!
So I just accept it the way it is. I don't mind.
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Edo
(2003 Silver 1.4 Tdi Exclusive(SE), black interior, sport seats, heated mirrors, JVC KD-SH909RB cd-receiver, Infinity Kappa 63.1 + Vifa on-dash tweeters, 12" Subwoofer on 300w Amp)