The head unit has already been replaced, the USB cable replaced (twice) and some cable behind the dash as well. Behavior the same in all cases. If we get a really warm day (rare in Seattle) it starts working for a few days, then stops working. If it has been warm for a while, and then gets suddenly cold, i.e. overnight; then it starts working again for a few days.The USB is not grounded to the body of you car. It's grounded to the sound system. Which is grounded to the vehicle. The sound system has circuit boards, full of microprocessors. What are you planning to try or fix? Or shall we say. Fry or burn up. Your think you can ground a USB source to 12 volts. There very little power running to that USB. Very little.
Pull the radio, take it to someone that repairs them.
So, it seems like a continuity issue to me.
The question is, where?