Hi,
The problem is that often the chassis of the speaker is earthed (not on purpose, but just due to the fact it is a metal chassis bolted in car), the high and low frequency speakers come from the same amp output I believe, so if there was a connection problem it could be grounding the output.
This could also explain the shut down of the system. Does it do it when the radio is switch off, but the telephone etc is on.
Poggy