mmm...just a month ago, my son's Intrepid got a similar problem..in fact he had the problem for a while, but last month, we decided it was enough. After lot of testing, we finally agreed to the idea we would have to dismantle the complete dash board...and we found a badly routed wire bundle that had finally chaffed against frame parts and made a solid short circuit. We do not know if it was like that since the time it was built or if the fact that before my son fgot it, a remote starter had been installed and lots of things had been tampered with.
SO...this is a possibility....but also, since the radio is involved, notice that speaker wires on modern radios are both HOT, and they are sometimes tampered with...so inspect speaker wires too, at least, for a start.