these truck radios ain't hi fi home stereos

audio is an AC signal of varying phase or "polarity", not DC - a truck speaker has 2 connections, one [either one] goes to a jack or terminal on the radio, one [the 'other' one] goes to the radio case or "common", usually using a "ground" near the speaker if the radio is mounted to the metal cab - as wrenchbender sez, "polarity" will have an effect with respect to stereo sound .... if you have multiple speakers they should all be hooked up the same or the "out of phase" condition created makes for dull sound .... OTOH, the original speakers in these trucks were not marked + and - as it made no difference whatever for the simple non-stereo system - the cone pulls in when the audio waveform signal tells it to, pushes out for the same reason, and you want multiple speakers pushing and pulling at the same time

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics