Which wire for LED load resistor?

[4RunnerTK]

Good morning, I'm installing LED light bulbs in my 2013 4Runner SR5. For the rear turn signals, there are multiple wires leading to the bulb and no ground wire. It seems it grounds by the socket when it's installed into the metal body. But, which wire leading to the bulb is the main positive that I should splice the load resistor into? And do I ground the other end of the resistor to the metal body because there is no negative wire?

Thanks,
TK