Only have high-beams

Yup. Usually it is because someone wired the aftermarket radio incorrectly. To be specific, there is a tail light wire going to the radio to tell the display to dim at night when the head lights are turned on. There is also a dash light wire going to the radio from the dimmer rheostat to tell the display how much to dim.

To add to the confusion, there is no ground wire in the radio's plugs. The ground comes from the wire or braided strap bolted to the back of the radio. When uninformed installers search for a ground wire, they mistakenly find the tail light or dash light wire because they read such low resistance to ground through their many bulbs on those circuits. Using either of those wires for a ground wire shorts that wire to ground through the case of the radio.

The place to start is by unplugging the radio, then working on the lighting problems. Once those are solved, reconnect the radio, then see if any problems occur again.