If your car is going to sit outside in the sun most of the day you'll only need something that puts out 10W or so (Ebay one is a bit small). Make sure it has a charge regulator as well (Can be inbuilt, but mostly sold seperately).

If you want a flexible panel (generally more expensive, but can be thrown in anywhere) give the 12 Volt Shop a call. They are up Kewdale way. Ask for flexible Uni Solar panels (not sure if they make these anymore though).

But otherwise, any no-name brand panel from Jaycar (5 - 10W) should do the trick.

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TD06 - Oink.