clio cup rear speakers..

[Phill1978]

knowone sits in the back as they would roll around to much (no seats) so i have two speakers that up close sound really really shitty. Could i get 13cm Bass cones in there ?? this would beef up the sound without adding the weight of a woofer + amp and then i could improve the front mids to decent speakers (the cups are worse speakers than 172 im sure of it) i already have 50 x 4 watt sony headunit so im ok on the power side.

what mid/bass (basically bass units) would you recommend to replaced the rear 13cm speakers in my cup.. (they are 13cm`s arent they ??)