If you think about it, why stick a bike on the back of any vehicle especially this time of year? a couple of miles down the road and it will be black and caked up with salt and muck, then you have to man handle it off, its filthy, sit on it and hold the handlebars also filthy.
I think your right when you say you can see the lights maybe through the wheels of one bike maybe if they are in the right place but Ive followed cars with up to 4 bikes one on top of the other on the rear, all different sizes and you cant tell what make the car is let alone see any lights properly.
Plus the rear of the car gets scratched paintwork into the bargain.
Its a case of youve got a car but really need a van/minibus so you can stick them inside, either that or get folding bikes that will go in the boot then hang the misses handbags on the back