Not all radios "suffer" from this out of intended use issue; if your drain goes away with the radio isolated, then like our Pioneer your probably does.Was just chatting to father in law and saying i would add one of these. We don't use the radio but if we came to sell the van, would like it there still so this seems like the best option!
The "memory retention" in our Pioneer [2007] only takes one or two mA, so that element is no issue at all.
The "dormant" radio was a whopping 600mA on its power feed, that was our issue that pulled the battery down. It is also a DVD reader, not just a basic radio. Not all radio aerials are "passive" some are powered, another potential drain if the installation was not well executed.
Edit: probably best not to place too high a possibility it is the radio, however it is very simply isolated by extracting it and pulling its power feed plug(s) from the rear.
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