The Goldpoint SA4 Passive Preamp Schematic

  This unique passive preamp allows the LINE IN 4 (AUX) RCA jacks to be used as an 
     Auxiliary Input or Output by changing the position of one wire jumper per channel.
     Line IN 4 (AUX) can be set to be a Line Input, a Tape Output or a 2nd Main Output.

Another Passive Line Stage Schematic

If you set the MODE switch to position 1, you can hear the SOURCE selection, but the recording output is not connected.
When the MODE switch is set to position 2, you have Mute, with both the Main Outputs and the Recording Outputs disconnected.
You can listen to what is selected by the SOURCE switch at the same time you are recording it if position 3 of the MODE switch is selected.
The Grounding switch feature is sometimes useful when a system has "ground hum" problems (but this is not typical). The Grounding switch can be a Double Pole, Double Throw, Center OFF toggle switch. In the Center OFF position, the grounds from each channel (Left and Right) are isolated from each other. Flipping the toggle switch up causes the two grounds, Left and Right channel grounds, to be connected to each other. Flipping it downward causes both grounds to be connected to each other AND connects them both to the chassis (if the chassis is a metal one, which is normally recommended.)
If you do not need a Ground switch for your system, you can skip using one, of course. Simply connect the ground wire from each channel together (as if shorting across the two center points (lugs) of the Grounding switch shown below.) 
For that matter, you can skip having a MODE switch too, if you wish. Simply connect the 1K resistors to their RCA jacks so you would always be able to record what you are listening to - or delete the 1K resistors AND their RCA jacks from the circuit entirely.

The SOURCE selector switch shown is a 2 pole, 6 position. This could be a Goldpoint  P/N 2P-6T-1D (or modify this to however many inputs you want to have, such as 
4 positions - Goldpoint P/N 2P-4T-1D.)
The MODE switch is a 4 pole, 3 position, such as the Goldpoint P/N 4P-3T-1D
The GROUNDING switch shown below could be a double pole, center OFF toggle switch. Alternatively, it could be a 2 pole, 3 position rotary switch, such as the Goldpoint 
P/N 2P-3T-1D, (or, likely can simply skip using a grounding switch.)