Here is something you can easily setup to keep your radio on with the key off till your door opens. This is the diagram for cars that use a door switch that controls ground, which I believe is what most or all of are cars use. You do not have to use the diodes, I think it is just a type of protection. There is more diagrams here for different setups, http://www.the12volt.com/relays/page5.asp#drto
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