My daughter has never tried to get out of the 5 point harness while driving, but we have a high back booster in one car that is secured by the shoulder belt. If your daughter is big enough (I think height is the most important factor for proper placement of the shoulder belt) you might try getting one of these. I didn't shop for it with her (which you could certainly do), but she was super excited about her new big girl car seat. She didn't unbuckle it during driving, but she did unbuckle it once when I was pulling into the drive way (I had already made it absolutely clear that she would not be permitted to unbuckle it w/o my permission because it was not safe and we must always be buckled in, plus it's the law) before I had stopped. I immediately took the new seat away and returned her to the convertible car seat w/ the 5 point harness and told that if she couldn't act like a big girl, she couldn't have a big girl car seat. Worked like a charm. I kept her in the other for a week or so and since she's gotten it back no problems whatsoever. She also likes it (as do I) b/c it's easier for her to get into by herself b/c it's not as high up on the seat. Just make sure you pull the seat belt all the way out and then let the slack go back in after you buckle (as you would if installing a convertible seat) so that she does not have the freedom of movement from the extra slack (hopefully that makes sense). I think there are also guards, and you could buy one that is harder to unbuckle, but this is one of those areas where I believe it is better to foster the idea that it is not okay to try to get out. Just buying something harder to open only encourages them to try harder (hopefully) w/ less success but does not teach them to comply with the fundamental and absolute necessity of being buckled in and safe.
This is definitely one to take a stand on and make sure that she knows who is in charge - personally, something like this is an absolute mandatory must and I don't believe in rewarding a child for complying with the law - she should no that this is 100% nonnegotiable and that her compliance is expected, not something to be bargained for.
You might also see if there is an Elmo's world or a book of some kind that talks about safety and seat belts that you can read together. Taking her to a safety seat fair or arranging a visit w/ your local fireman or police officer may help also.
Here's the seat we purchased:
http://www.toysrus.com/product/index.jsp?productId=2899788