Green’s Dictionary of Slang

(orig. US) a citizens’ band radio, its antenna or the vehicle carrying it; thus have one’s ears on, to be tuned into one’s CB transceiver.

1975		L. Dills CB Slanguage 36: Ears: CB radio; duel [sic] antennas [...] Ears On: CB radio turned on; e.g. ‘Got your ears on, good buddy?’.
1977		C. McFadden Serial 72: Any ears out there? [...] I gotta get me a CB.