Proximity sensor is STILL not working

I bought this phone off eBay knowing that the camera doesn't work and it came without a screen... So I basically replaced everything in it... Including the camera to see if it would work (it didn't ) , the one issue that has always annoyed me greatly is the proximity sensor, it's so annoying the way it doesn't black out when I'm making a call and I always hit the mute button with my face.

I have even bought the little sticker that says it fixes the problem but it still does nothing . I'm also on the latest firmware 5.1.1

I also tried using scotch tape to cover up the sensor then coloured it with black marker .

Nothing has worked . I've also tried completely replacing the proximity cable twice and still nothing , any ideas ? I'm on the verge of selling this if I can't fix it, thanks