Im working on a similar project. Ur radio looks a lot like mine. The Amp will seperate from the tuner and there's a nice surprise on the top of it. Sorry about the tape, used some crayon to show that the pinout is etched into the amp. I was just about to take a stab in the dark when i noticed it through the thick layer of dust.
So, basically, you could splice into the back of the unit, if you wanted to instead of soldering. But, the pinout is worth finding.
Oh, you probably already know, but, the terminals are marked on the female connector on the back. One corner has, in my case, a 16 and the other a 1. The other connector has two rows.