my personal opinion would be dont bother. They normally arent done correctly and look tacky as....
but if u are determined, then dont buy them from halfords as they bloody cost a mint compared to Maplins. Their like a fiver each. My opinion wuld be install them where the ey cant see the tube itself, one in each footwell upfront, under the trim tacking them or glueing them or even tieing to stop them from moving. As for the back i cant think of a better way of pimping ya car out than a curtousy light replacement bulb in the ceiling, as there are no real ideal places apart from under the front seats, and theyll probaaly get kicked and shortd or broke etc with feet and seats moving back and forth.
YOU CAN EITHER TAKE OUT THE BULBS AND BUY GLASS PAINT, OR if u want to go to the extreme, fit some led;s in there, some extreme intense ones, or better again, to really stand out in a croud, buy some ultraviolet sensitive tints, installed by a professional ofcourse, and an uv light in te ceiling, and the black tints will turn white everytime the light comeson, and u could connct that up to the bass in your car.
if there aint enuf ideas there to get u stimulated, then i give up. Nevertheless, i would still say dont bother, subtle it always better.

good luck with it though