Truck rear turn signal and stop on different wire?

[frameless1]

Hi guys I have a question about the turn signal and stop or brake light, why are they on the same wire to the rear of truck? And more importantly how can I get them separated? The reason being or the short way of explaining is that I would like to get an amber turn light and a red brake light to work independently of each other. I know that on the back of trailer there are wires for each of these functions and I know I could possibly hack into the 7way cord at the end of truck cab for me to run a cable to rear of truck frame for the function I need but still intrigued as to why are both signals run in one wire? Is it just to make it cheaper or does it serve a function that I don’t know? Just for not leaving it out I’m talking about a 2013 Cascadia [/align][/size][/font]