Simple truth is be brutal and only take what you really need, does your girlfriend really need to take 10 pairs of shoes, 3 handbags and more products than boots the chemist. As for clothes take the bear minimum and take half a day out somewhere and find a laundrette, admittedly not what you want to do in the middle of a holiday but a damm site easier than trying to cramp 2 weeks of clothes into the car.
I don't know about you but stuff in the passenger footwell would drive me mad even if I was driving, If I can't get it in the boot then doesn't get packed, generally speaking. The good lady wifes handbag being about the only exception to the rule.
As an aside I used to go away camping for a couple of weeks at a time while touring Europe on a Motorbike and managed to carry everything I needed.
Don't forget if you lift up the tray in the boot (roll the knobs back) you won't be able to lower the roof.