Did you ever figure this out?

 

Not really I just had the dealership install the dedicated cable for storing it so I just have to plug it in.

 

I got my LS500 this afternoon and they had it hooked up to a battery charger for the duration of the delivery process. I know where to hook it up but it doesn't look like I'll be able to hardwire it that way. I guess we'll see. My car will sit for weeks at a time sometimes so I will look into it. I think the official stance is "don't do it" because they don't want you messing with things.

 

I've used battery tenders for years on my convertibles that I seldom drive during the winter. It's pretty easy to get the extension cord and the standard 12v connection plug, and easy to hook it up to the + terminal and a ground. All my batteries were under the hood, so I ran the cord up to the space under the windshield wipers so that there was no need to pop the hood. For a battery in the trunk you will have to be a little more creative, and maybe just leave the plug and the cord in the trunk itself, but you should be able to close the trunk with the cord hanging out. The extension cord with the plug on it is a great way to go. Highly recommended.

 

Woohoo I found out the previous owner installed a battery tender so I’m good. It runs to the front fuse box under the hood.

 

That's good news. I have noticed that on many cars with batteries in the trunk or under the back seat (Audi 100) that there is a direct wire to a positive terminal under the hood, to enable convenient jump starting (finding a ground under the hood is never a problem). This would be an easier connection for a Battery Tender external plug than trying to connect it to the + terminal on the battery in the trunk.