Hey Andy
Ok, I’ll see if I understand this. You have the two lights on each side at the back and you want to use both of them for your turn/brake lights. If this is where you are going with this, you will need the relays to handle the amp load safely and diodes to prevent the current from backing through the circuit and trying to run everything. This sounds like a mess, but if I can do it, anyone can. It’s easier than it sounds.
I’m pretty sure I have this wiring diagram and I will look it up when I get some time today. I’m at work right now and I’m going to be tied up for a while today.
J.D.