Or I could buy a new head unit that doesn't need a continuous power supply.... !
This might be a daft idea, but could it be cheaper maybe to buy a small 12V power pack with the output connected to the head unit and the input wired to a switched live feed. That way it would keep the head unit alive when the car's parked and the power pack would be recharged when driving. So long as you had a power pack with sufficient capacity to keep the radio fed for the longest period you would leave the car without either driving it or plugging in a battery conditioner you should be fine.
