Hi, i have a 3dr vauxhall corsa, i can’t access the rear speakers? I’m trying to connect wires from a speaker shelf but can’t.

Hey, I have been looking to upgrade my stock Corsa speakers and am having a little trouble finding some.. this is due to me not having a clue. When I bought the car it already had a new head unit in it, it's a Sony Xplod something... It says 52Wx4. Do you guys have any suggestions what...