To help keep the battery topped up, I thought of putting a 10W solar panel on the rear parcel shelf.
The obvious route for the wiring for me is through the parcel shelf (? Bass ?) speaker.

It has no speaker and a rubber sheet in there.
Is there a way to partially remove the rubber without damaging it?

Or is there a better location for passing through the wiring?

I’m having difficulty (temporarily) investigating due to a bad back.

 

The right side behind the tank cover panel has a foam covered hole that should let you run the wire over to the blanking plate for you speaker. Might be able to just run it on top of the tank. There is a wiring harness there that should allow you to zip tie it to secure the charger wires. The metal cover has 5 10MM bolts holding it in place.
Strange it doesn't have a speaker there, but people do funny things.
Good idea by the way, if your car sets outside.

 

That is one project that I doubt will pay dividends. I would save your back and just attach a battery tender under the hood (bonnet) and keep it plugged in when not driving. I just came back from the UK and I doubt there is enough sun to charge a cell phone, was there during storm Brendan. Even with the sun shinning you would have to make sure it was parked at the right angle for it to work properly, and even then it might only be for a few hours. Think you would get more power installing a wind generator on the front grill.

 

I have a convertible so don't have any direct experience of handling the coupe's rear parcel shelf below the rear windscreen. Was browsing on the websites and saw this.


So fitting something through the rear parcel shelf blanking cover looks straight forward. As cjd777 said, there should be a good hole to thread cables through on right side fender by the tank. In my convertible, all my hydraulic cables go through this one big hole. From there, getting to the battery would be easy.

Having been to Wales (was wet and windy), I would second MiragePro's sentiment, that you probably won't get enough power from the solar panel though - remember you need enough voltage and current to overcome a lot of resistance on car batteries to charge them.

 

Also you can follow the wring harnesses which run through an opening under the parcel shelf and behind the rear interior side panels. It is fiddly but it would be a neater route to the battery. This what I did when I installed a new stereo and amplifier.

I had 30w solar panel on the roof of a garage (with no mains power) that I used one winter and that kept the battery topped up OK.

 

Thanks for the input.
My back is a bit better today, enough for me to take another look.

I did indeed go through the blanking cover and was easy enough to do even with a dodgy back.

I have used this technique to keep my Audi A8 (similar size battery) topped up whilst I went travelling for 3 months. It worked.
In fact, at the moment, I have the Audi battery (90Ah) in the XK8 now as the Varta (100Ah) that came with it is dropping to about 12.5V after a day (sat in the house), after being fully charged. Maybe its goosed.

Obviously, being hooked up to a trickle charger would be much better, but its not an option for me.

And yes, having the solar panel angled to the sun is important. The parcel shelf in the XK8 doesn’t lend itself to the right angle either. So it will need to be propped up.
A quick measurement, not connected to the battery, lying flat, showed 17V .
In the Audi I rested it on a rear headrest and it was at good angle, facing the sun.

I have read that the XK8 does seem to have quite a draw on the battery even just sat around.
If I have got this right, then it does go into a sleep and eventually a hibernation mode.

I have only had the car for less than 2 weeks, so have a lot to learn.
Thanks again for your help.