Tweaking Advice Needed

I have a young relative that has installed his own system in a single cab Ford Ranger, which he's done a pretty decent job of doing, but is now in need of some tweaking to get the most out of it. He has the following equipment installed (sorry I do not have specific model numbers):

Sony HU, Fosgate Punch 45 amp, 2 Kicker DVC sub's, 2 Pioneer 3 way 6x9's and 2 Pioneer 2 way 5.25's. The Kickers are installed in a box and located directly behind his seats on the floor. The 6x9's are installed where the OEM 5x7's where located and the 5.25's are located in the doors. The amp is bridged and pushing the kickers only. The HU is pushing the other 4 speakers. Obviously, the bass is overwhelming as the builtin HU amp does not have enough to make more than a squeak come out of the 6x9's and 5.25's. Money does matter in this case, so buying an additional amp (mono amp) to push the sub's is out of the question for the time being. Can anyone please recommend a method to wire the other 4 speakers into the amp along with the subs? Is this possible or trouble waiting to happen? What other equipment is required? Please, be specific as I am really out of practice and need to be able to relate to him in a very straight forward way to do this.

Thanks,

Razorbacx