I have a Ctek MXS 5.0 Battery tender for my 2011 XFR and upon reading the instructional booklet, it states that if the battery is still connected to the vehicle, to put the positive cable on the positive battery terminal, but to connect the negative cable to a ground point on car( not directly to the negative terminal). Being that the battery is in the trunk of this vehicle, are there any ground points in the trunk I can use?

 

Yep, on both my XFS and XFR I connected the negative lead to the battery tie down rod bolt on the side of the boot/trunk, worked a charm.
My battery tender (not a CTEK) came with two sets of leads, one alligator clips and one pigtails.
I bolted the round washer ends of the pigtail leads as above, then ran the cable under the carpet/bootliner on the side of the boot and poked the pigtail end out the bottom of the fuse box cover panel on the RHS of the boot. The cable juuust reached far enough. It was then a simple matter of plugging in the pigtail whenever I used the tender, no fuss no muss.

 

You can also put the negative terminal onto the spare tire hold down bolt.