The single input is meant for a system that will send a mix
of both channels to your sub unit. You can do this by sending
an Auxiliary line level stereo signal to the unit. First, you will
have to build a special Y connector or splice up a cable so you
can tie both channels together. Cut a stereo patch cable in two.
Strip each end, exposing the conductor, a little of the center
conductor's insulation and the shield. Tie the shields together.
Take 2 resistors (500 Ohms at 1/4 Watt) and solder each one
to it's own signal wire. Tie the other end of the resistors together.
The end of the tied resistors is your mixed signal out to your
sub unit. Solder one of the other ends of the patch cord to the
tied end of the resistors. Tape all of this up with electrical tape,
using small pieces of tape just enough so that there are only
two layers and no short circuits to ground (the shield). Tie the
shields together. This may require a small piece of wire to connect
the single shield to the stereo shields. Don't worry as the shields
are all common to one another inside all stereo systems. Wrap
up the rest of the assembly in one long piece of tape in a spiral
on over all your work. You are now ready to plug into your sub
unit. The resistors are used so that you don't short the right and
the left channels directly together, possibly affecting the entire
system. Rock on, brother!