I have one in transit from the vendor.

Is it better to wire it in the trunk, or in the engine compartment?

I presume there is an accessible fuse block, under the hood, with a place to stow the bit of wiring which leads to the plug.

 

It might be safer to mount it near your battery and use the fuse box located in the same area. There are several mounting sites within the "spare tire" area that may serve better then within the engine compartment.

I used a solar charger for about a year and it worked fine. kept the battery charged while I was deployed. The solar charger worked fine within my garage, using the florescent lights as a power source.

 

I have a CTEK for my XJR, use the cable with the quick connect that comes with it directly onto the battery. You can close the trunk on the cable & have the charger outside. Means the car can be locked up & you can also see the status lights on the charger. The quick connect on the cable means you just have to unplug it & away you go.