Trying to install my new radio but can’t find one of the plugs, my FSM wiring book shows R1, R2, & R3. After removing the old radio (hooked to R2, gray plug) I found only one other plug in that harness and it is R1 (I believe) for the front speakers, R3 for the back speakers is missing. By chance is R3 on another wiring harness that may be hiding in there, or is this normal?