Grant, i read your explanation a few times and still can't picture what you have done and how... you spliced your interface (Resler iBus?) into the iBus and power of the car? the USB has 4 connectors, 2 connectors are data, one is (+) and the other one is (-/ground) 5VDC. if the other end of the interface has 3 wires, that means that you have 2 for data, and the other one is probably for ground? could it be that the other one is (+)? i think the issue could be with one of those "power" wires...