On a version 2.8, the wires for external lights are purple with a red stripe and purple with a black stripe. See the Dozuki guide #4 for OROV 2.8, step #90.
You can certainly try just putting a bare 9V LED on the wires. I suspect it will be bright when the batteries are fresh, but it will dim when you run the motors hard, and will cut out altogether when the batteries get low. You should check it with an ammeter to ensure that the LED does not draw more than 2A when the batteries are fresh.
On the OROV 2.8, all the necessary wires are pre-connected inside, so you don’t have to do anything other than hook up the LED. The latest software (30.0.3) fully supports external LEDs, using ( I believe) the Alt-I, Alt-O, and Alt-P keys.