putting 98 head unit in 04?

in my 04 gt, i dont have the mach system stock or nothing. so I went and got the back speakers part that has the whole amp deal all connected from a buddy. Its out a 95... I have 2 questions....I found a few 98 cassette and cd slave deals for a cheap price. Would that little unit fir into my 04 fine? Also, would that 95 mach system in back plug into the back of the 98 head unit deal? because i dont have the same plug on the back of my 04 unit? Thanks, the ebay thing over in about day so any help fast is greatly appreciated.. Thanks

i only got the mach speak and back deck stuff because it was free so i was like why not.. I dont really care to much about all the audio and stuff but i figured may as well try and get it to work if its only going to cost me a few bucks ya know?