Does anyone know the "trick" to unlock the DVD door? I tried
everything! The mechanism doesn't even try to move, the lock sigh comes
on right away! Thanks to Andrew for the info, but I still can't get the
unit to open! Dani.

RCA Model # DRC 500 DVD changer. Works fine, but when you press the
eject button, a red "lock" appears in the display. Nothing on the RCA
warranty site. What's the trick to resetting this problem? I have tried
holding down the eject (either on the remote or the player) for several
seconds? Thanks, Dani.

According to the manual for several RCA DVD players:

Press and hold the STOP key on the DVD player and press and hold the
ON/OFF button on the remote (make sure to aim at the display). Keep
holding them down for at least 2 seconds.

From another model:
To lock/unlock, press SETUP 0 0 0 8 3