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Check the entire length of the cable or wiring to the sonar transponder, looking for strains breaks or severe bending. Clean all connections to it from and at the Garmin 75, one at a time: disconnect, scrub each side of the connection with a clean dry brush, then wipe until the metal shines, then re-connect firmly, and on to the next one.
Then try again.
Red usually is + and black - voltage, where power connections are concerned. But what transponder do you have, what maker and model ? And did you hand-wire the connection from the sonar transponder?
Check, also, whether the sonar transponder is still firmly mounted and undamaged (by perhaps, collision, or grounding, etc.)