Hi Rohan
50V is too high voltage. It is good that you decided not to connect it.
About USB problem. Is there only connection problem or pcb also doesnt convert signal ?
If there only USB connection problem, please check usb cable and connector on PCB with multimeter. Also please check voltage between 1-2 pins of the service connector( with supply voltage /ignition switched on, engine stopped)
http://max.pilotpowersupply.com/wp-c...EN-300x154.jpg
there should be 5v.
If all ok, then it seems there is a problem with pl2303 interface. You can try to resolder pins of it. Sometimes it helps. Also if you can buy bluetooth HC-06 modules in your local electonic shops( or can order it on internet), i can give you instructions how to solder it to your converter and you will get bluetooth connection with pcb without any wires.