Welcome Meeson!
Perhaps you are aware of this but contrary to your diagram, the factory head unit has four speaker outputs:
Front Left and Right
Rear Left and Right
The dash 4" mid-range and tweeters along with the feed to the amp in the trunk that drives the door speakers are all connected to the HU front speaker output.
The rear seat 4" speakers (identical to the dash 4" by the way) are driven by the HU rear speaker output.
Check out this post for more info:
https://www.ft86club.com/forums/show...7&postcount=35