I made like a cubby box thing for the middle seat, had a radio from marlins i think it was and I got 2 speakers off eBay that had there own box, cut the cubby box to suit the speakers and radio, wires went up the side of the back window(mine is a pick up) and around the top channel, down the side of the windscreen(on the inside) through to the back of the dash and then through the bulk head were other wires like the fuel cut off go and speedo cable etc, went onto the battery for the constant live and I think it may have been the wiper switch that I went onto so that the radio turned on when I fired the beast up, was a total of bout 80-90£ but the radio was like 70, it was one that was far to high tech but I wanted to get it sorted so just got it, speakers were I think £12, plus you need some wire and crimps..... (having the circuit diagram helps a lot for wiring) I'm 15 and got it sorted so can't be that hard.
Hope I helped
Ben