4 Wheeler anteanna mount location

Ok here is the situation....
I'll be starting CDL school here shortly...I had a CB years ago...and want to install one on my car for the daily drive to school...2 hrs each way (4 hrs a day) for 6 weeks.....
The car I'll be using is my MINI cooper S...with a Sunroof, so a mag-mount will not work very well..there is only about a 4 inch band of roof......the top is like 90% glass (I do/have driven larger cars, pickups, and even a 6 wheelers so no ridiculing please!! It is an honest FUN to drive car, with a custom dino tune,bigger injectors, big sway-bar, etc, so it is a blast to drive)......and I'd like to avoid a glass mount...they simply loose too much reception.
I'm Thinking of mounting it to the rear bumper...putting a Fiberglass Francis antenna on the tow hook mount (I'll make some type of mirror mount, etc work on the oval ring...it gets screwed into the bumper through a hole and is only mounted when needed..so grounding should be fine)....
So this is my question...do I need to get a ground plane kit or use a different antenna to make this setup work? I'm planning on getting a long enough antenna to it will stick up above the roof line...I actually toyed with getting a 102' SS whip...but the car would look comically like a remote controlled car!!
At this point I think I'll be getting a Cobra 19 from Sparky's...I would love a bigger radio, but the space for mounting is tight, Money is tighter, and the 19 seems like a good starter for a bit...any suggestions to improve the set-up or any other ideas? The glass mount would be nicer...But I have always heard they are a BIG compromise. Have they gotten any-better?

 

Ok...here is pic...not of mine...I have a Hard-top, but this is the mounting location.

 

yeah....might get the SS 102 whip....Been thinking about it...they are cheap, effective. and locally available...heck the ONLY antenna at the Rat-shack (Radio-Shack)that is worth any $$ in my opinion. I was afraid it might bounce a bit too much...so I was thinking of going with a 5.5 or a 5 foot fiberglass...to try to get as much as I could above the roof (will be close to 60% anyway).
I guess it might look kinda funny/cool/silly to look like a bumper car with the 102 whip! Might five folks something to talk about!!

 

The mounting location would be damage free...there is a steel O-hook with a threaded bolt that goes into a factory location in the frame..it is for towing the car , and they are run with them both on, the front and back on track days...during normal driving, there is a body colored plug that covers the hole. Heck, the battery is in the trunk...so I even have a factory hole, with a rubber grommet in the trunk to get the cable out. The location SHOULD be a terrific ground....just no ground plane.....hence the question on getting a ground plane kit or not. I guess I'll go without...and see how it works.