Black screen pops up states caution your screen may seem reversed

I'm assuming this is a DVD player or at least contains a video screen. The message you are seeing sounds like the rear camera input has been activated. They give this alert to inform drivers that the view through the rear cam may be reversed. Most cameras have a switch setting to allow this adjustment.

If you do not have a rear camera installed, then it sounds to me like the rear camera trigger has been improperly connected. This is a wire that when it sees a 12-volt signal, activate the rear cam input on the radio. If you do not have a rear camera, this wire should not be connected. You did not give a make/model so I cannot begin to guess the wire color, but look for a wire that is not either black, red, yellow, white, gray, green, violet, or blue (with or without stripes). There is no standard color for this wire. Some Alpine units use orange/white, but on most radios, this is a dimmer wire. Pioneer and Kenwood often use a light purple or purple/white.

Find out what wire is for reverse camera and disconnect it.