How many speakers do you have now? I can send you pictures of the 2ch and the 4ch amp. From what I see in my 760 the po bypassed the amp and wired the stereo direstly into the speaker wires (4speakers) I spliced the dash ones in when I bought it. Crutchfield sells a harness so you wire your new Head unit into the harness and then plug that into the OEM connector that was plugged into the back of your unit. The dash speakers are powered directly from the Hed unit so I am not sure how you'd work that.

I like the factory head unit it sounds really nice with better speakers, though the bass is off. I have speakers in the front doors, rear doors and in the dash. I wanted a little extra bass from a sub in the trunk hence the need for the extra output from an additional factory amp. I listen to classic rock, jazz and kids music. So I don't want/need the car next to me to get a head ache from the thumpin bass in my car, I just want what is missing from Phil Lesh's bass solo.

PS I have a wiring diagram for a 740 stereo. Let me know if you want it I'll zap it to you.