If the existing radio is more modern it will likely have one or two multi-pin plugs on the back - two if it is capable of driving four speakers (i.e. has a fader control) or maybe just one if not. DO NOT assume these are to standard wiring even though the plugs indicate use of ISO connections that you can easily find on the Interweb. For instance a radio designed to be fitted in a VAG vehicle will usually have the permanent (memory) wire and the switched wire reversed, and traditionally that group feeds power to the aerial amp up the coax, not separately as most other manufacturers (although many such as Vauxhall are now going the same way.) Also remember in rewiring that when you hitch up the habitation relay in the 'van will usually disconnect the internal 12V; if the memory wire is from the standard 12V supply then all radio memory contents will be lost - consider running a separate wire from the battery to the radio to keep the memory. That line only draws microamps, but for winterisation you could put a switch in the line or simply disconnect the battery.