The cable's (PL-2303 chip) output level is 3.3V, but the voltage level of DTR pin at your cable version is unknown! (it must be 3.3V, when it is not low)
Since there is many different cable version and PCB layout of PL-2303 chip, every cable hack depend on the very same kind cable and PCB layout of the different PL-2303 chip.
A new approach:
The DTR pin Auto-Reset the Arduino (when uploading a sketch) by pulling the reset pin low (0V) that's way you are going to do the same as you did, but only this time set the multimeter to find the cannectivity. This time look after which pad/pin will be connected to ground(0V) when you are uploading a sketch, you have to be fast, it is only for a few seconds. (it would be more easy if your multimeter has a buzzer)