How to get into the trunk of a 65 Barracuda????

Pop a body plug out of the trunk floor or spare tire well. If you're lucky, You have enough access (I.e. spare tire mia) to reach in with something to trip the lock from the inside.

If you're not lucky, you get five gallons of rusty water in the face.

Fyi electric lock releases from m bodies, diplomat/5th ave, should fit that car.

If there's a split/gap between/above/under the rear seats you may be able to guide a REALLY long flat blade screwdriver or equivalent between the seats and engage the latch that way. Try pulling the lower seat bun first.