so wouldn't a no ground plane antenna work better then? what about the left rear above the tail light and having a post of some sort extend the antenna above the fiberglass top? Also, is there much performance difference between a 1/4, 1/2, and full wave antenna considering the constraints from a Jeep?
No ground plane is a misleading concept. What it means is the antenna does not
need the ground plane (e.g. the metal of your truck under and around the mount) to achieve a suitable match to your coax feedline. NGP does
not give you the ability to just put the antenna anywhere and automatically expect good performance. A bad location is still a bad location even if you achieve a low SWR.
There's two very basic steps here. First to get the energy from the radio to the antenna. If NGP is necessary to achieve that then you do what you need to. Having a ground plane like a big metal roof is nice in that it gives you flexibility but you can work around that. All mobile radio is a compromise at some level.
When thinking about this don't forget that there's more than just a whip over a plane. You can use many things as a counterpoise to achieve a good match. Running a wire in the opposite direction as the whip can do it, essentially making a dipole turned vertical. But metal in bumpers or racks can be turned into counterpoise if you're creative.
The second step after you've got energy to the antenna is you need the antenna to radiate it as efficiently as possible before it completes it's path back to the radio, which is its ultimate goal. Whatever you've referenced the radio to is that path, almost always the body of the truck.
If you put the antenna right next to the body on a tail light it'll be happy to complete the circuit with the least work possible. But you've not excited many of the electrons around it to actually send a signal anywhere. That's where putting up high and clear of stuff so you don't short cut or absorb RF comes into play. This is why on the lip of the hood usually will work better than a tail light.
It's also why you'll see people say SWR isn't everything. You can achieve an almost perfect SWR without the RF doing any useful RF work. A dummy load gives you perfect SWR but all it does is make heat. Dumping 100 watts into a dummy load might not even give you a radio path to your neighbor. While 1 watt into a great antenna that measures 2:1 SWR can work several states away.
Imagine the RF energy coming off the antenna as though it was light or sound or water. If there's "stuff" around that would prevent those things from reaching you then it's likely it will for RF, too. That's not always true but it's safe to say it'll be
extremely rare that anything around the antenna is going to improve performance. If you're lucky the materials around it are invisible to the wavelength you're using, but I wouldn't count on that being true.
ETA: Just realized I never addressed your second question. Short answer is it doesn't make
that much of a difference.
First thing is to not bother with a full wavelength antenna. There's no advantages that make it worthwhile. Stick with standard configurations, 1/4λ, 1/2λ and 5/8λ or typical combinations of those, such as collinears.
On a Jeep you probably want to use 1/2λ antennas mainly for their NGP characteristics. That's about the first step of just finding a way to mount an antenna that allows you to transfer as much energy from the radio to coax to antenna. With a 1/4λ you have to have a good ground plane and middle of roofs are almost mandatory.
But beyond that there's propagation characteristics. If you were to use 1/4λ and give it a non-ideal ground it'll have a distorted radiation pattern in all likelihood. So by using an antenna designed for imperfect situations you can usually get decent matching and a reasonably good pattern.
It's most important to mount the antenna in a way that at least gives it clearance away from body panels and RTTs so you'll be exciting more of the aether than using your radio as a mini microwave oven to warm up your sleeping bags just a slight bit. If you have to use a tail light or rear tub corner or swing out then opt for taller antennas mounted as high up as possible. It's about exposing as much radiating surface to bright, clear daylight.
On the edge of the hood then it'll be less of an impact I reckon. In that case going with a taller antenna or one that increases gain will make some difference, but less so than the difference between a stubby antenna blocked by your truck. If you use a 1/4λ on your tail light it's going to be a very distinct difference front to rear. You might not hear a station 1/4 mile ahead of you but will reach some a few miles to the back. It'll probably be very obvious.
Any location other than the middle of the roof will do that but think about it in geometric sense. If you have an antenna on the hood lip draw a line from the middle of it towards your truck. Where it intersects will be where it's blocked.
So while the cab will block it the shadow it creates is smaller than if you put it right against the windshield. On the hood lip you might not hear someone 1/4 mile behind you but only if they're the same elevation or lower than you. If they're perhaps 10 feet higher in elevation and looking down on you even just slightly they could be 5 miles back and still talk.
Think about in your mind. Right at the base of the windshield the radiating angle is basically just a 90° pie wedge pointed forward. Now move the antenna a foot forward of the cowling and the blocked angle swings down 45° or 60° so a smaller sliver of pattern is blocked. The ideal is having a 180° half sphere but the only way that's possible is in the middle of a perfect roof.
That's what's know as having a "line of sight". Think of it like you're looking at each other with binoculars or a scope. If the two antennas can "see" each other then it's just a matter of simple signal strength (even 5 watts is likely to work). If they can't "see" each other then almost no amount of power is going to work.
I really appreciate the advice.