After years of suffering from the scourge of the Lexus battery, I have replaced the battery; had the system checked; been on long drives; placed it on trickle charge and it still would be flat after two days, I have finally found a working cure, I've fitted solar panels into the rear spoiler.
I used to have the panels on the dash, but as the sun moved round that reduced the input, so I came up with the idea of inserting them into the spoiler, they are slightly proud of the surface to eliminate the risk of water collection and leaks, they produce up to 8W and 20V on a reasonable day. Since fitting I haven't had a single problem with the battery. Cost, around £80 for the panels (which I removed from their cases) and regulator, plus wire and an inline fuse. I have the power outputs permanently live so it's just a matter of linking into the wire, when I can remove the side trim.