Any aftermarket camera will have the same 4 wires (when the plugs are cut off). 12V positive and negative. Then the two wires for the video signal. All you have to do is fine some aftermarket camera that you can physically get into the same spot. Might have to first tape it in place and than make sure it is right side up, aimed right, held firmly in place, than use some silicone to "glue" it in place. As long as the wiring is done right to make sure the connections are watertight it should just work.

Many car audio shops are hacks. A decent shop tech (not the idiot sales guy in the front) could easily figure this out.