Remote wire for subs on stock infinity radio

[Drake]

So I've done my research and have found out that if I want to wire my car for subs and an amp I will need to splice into the rear speakers for my RCAs, but I don't know where to get the remote wire from. Do I splice into the radio harness? Or do I splice into the amp harness? This 95's system is a whole lot different from the 99' I owned.