There's one for sale on Ebay ($250), and a few service manuals priced around $20.
Looking at the unit on Ebay, balance seems R/L instead of a 4-way fader.
Whether the 4 wires (non-power/ground) are for 2 speakers (powered & grounded to tape deck) or 4 (speakers themselves grounded to chassis vs tape deck, I don't know.
TEAC at the time had some pretty high-end reel-to-reel stuff - I don't recall their tape decks.
We ran stuff like Craig, Pioneer, and Muntz back then, IIRC.