1974 MG wiring, where do i start?

[Jordan Guess]

My brother is getting this neat little car as a gift and Ide like to put a working audio system in it for him before he picks it up. I just ripped out the old school head unit which consisted of just a cassete receiver and a FM tuner with a volume knob. On the back of it to my dismay, was absolutely nothing to what the normal head unit wiring of what I am used to. There was about a total of 4 wires connecting to the back of it, 2 of which for the speakers in the doors. then of course the antenna plug in for the fm. My question is this, how do I go about wiring a modern head unit in this old car with such little wiring? is there a diagram or tutorial somebody can refer me to that will have all my answers? Im to nooby to know what to do in this situation and really am only usefull when it comes to the color matching and crimp technique. your help is much appreciated and I will provide any information you may need as well, thanks!