Modding a Fender princeston chorus, having issues reading this part of the schematic
First off, if you used RCA to 1/4 inch phone plug adapters, you would greatly simplify your problem.

As to the connectors in the amp, note that three of the nine pins don't connect to anything.  So now you have six signals to play with.  Pin 1 is ground; there's no trickery there.  And the actual signal goes in on pin 2.

If you've ever seen a 1/4 inch jack, you would recognize the line with the little crook at the end as being the contact that meets the tip of the plug, and sits in the depression in the tip.  The part that you aren't familiar with is the switch that is built into the jack.  The line with the filled triangle at the end is common part of the switch.  The dotted line indicates that it is activated when the inserted plug pushes against the tip contact.  At that point, it contacts the other arrow instead of the one it was originally touching.

In this diagram, there is an arrangement where the 8 volt supply comes in on pin 4, connects through its switch to pin 5, and is routed down to the second jack's pin 5.  That second switch connects to pin 4, and subsequently down to resistor R1 and the INH signal.  Presumably, this is an "inhibit" feature, which keeps the amp silent when nothing is plugged in.  Inserting a plug into either jack will open that switch (actually, switch to the other side (#6) which has nothing connected to it),  interrupting the 8v source and signaling the amp to come alive.   

If I'm right, the amp won't work for you until you put a plug into one of the inputs.  Cut one off a bad cord if you can't think of anything else.  But here again is where an adapter makes your life simple.

There's a second switch in each jack operated by the second channel contact (as if it were stereo, but it's not even connected).  This is pushed by the barrel of the plug (since there's no "ring" contact on the plug).  Its function seems to be, for Input 1, to ground the input when not in use, and for Input 2, to divide the amplifier between the two sources when they are both plugged in.  I don't think the schematic shows the switch contacts properly; they are shown how they'd be if a plug was already inserted.