He means the circuit that puts out sound for one half of the
speakers - one half of the stereo signal. It's internal. My opinion,
though, is that you shouldn't rule out a speaker wiring problem just yet.

The "head unit" or receiver technically consists of a control pre-amp, tuner
(radio), amplifier, (and maybe a cassette player or CD player) all built
into one unit. A failure of a single component (transistor, resister,
capacitor, etc) on the left channel of the pre-amp or amplifier could be the
problem. You will likely have to replace the entire head unit (receiver).

It is possible that the problem is the speaker connection (as suggested by
another poster) coming out of the receiver (not likely that the connections
went bad where the wire connects TO the speakers on two different speakers
at the same time). But most "likely" the head unit is toast on a unit that
is that old.