There’s a connector (likely a round DIN) behind the dash between the radio and changer cable. There’s also a connector somewhere in the trunk linking the CD changer and cable.

The other option is to use an RF modulator. Not the kind that you plug into a 12V “cigar” socket, but the kind that you hard wire between the radio and antenna lead.

CHANGER — CONNECTOR — C-A-B-L-E — CONNECTOR* — RADIO

ANTENNA — CONNECTOR — C-A-B-L-E — CONNECTOR* — RADIO

The connectors marked * is where you’d put the extra input or RF modular.