Dual car battery system

Does anyone know how to rig up a dual car battery system with means of 12V switching relays, or what other suggestions might one have for a separate power supply in a vehicle ?

I think something from caravan systems probably best way to go. They have split charging circuits, so when you towing caravan the batteries inside the caravan charge up.

As above and easy to do.
Do a google and don't but from a caravan shop the relay is very cheep and a length of cable to the second battery location plus a shorter bit to switch the relay from ignition live.

Split charge systems are easy to wire up, but be mindful to either pick a supply point on the car that is only live when the engine is running or if going direct to the cars battery then you want to get a voltage sensitive relay so the second battery only charges when the engine is running ie voltage is higher than 13 volts.  Dedicated split charge relays can be expensive depending on charge current required.  I have used cheap voltage sensitive relays bought on Fleabay.  Otherwise there is the chance you can discharge both batteries, unlikely with a radio but you don't want to find out the hard way!