Amp setting - Input Mode

[EngineerD]

I'm setting up an old Alpine 3542 amp for a pair of components, what setting do I need for Input Mode?
Obviously not mono but ST (Stereo?) or L + R (Left & right?)? I can't any instructions for the amp online, just a wiring diagram.

 

[EngineerD]

Think I have found it, I need ST, L+R is summed.