A fuse could be bad underneath your dash
Because the power for your headlights is pulled from your battery. When the dashboard lights dim after turning on the headlights the dashboard lights aren't getting as much power as they were with the headlights off. With the motor running or off? The headlights draw a lot of current, but should not visibly affect other systems. There could be a problem with a worn out battery, an alternator that isn't charging correctly, or you have a bad earth between the battery and the chassis. Check the large cable connection between the battery (usually negative these days) and the bodywork, or between the engine block and the bodywork.
what makes my 2002 inside lights will work during the day but not night, my dashlight, speedometer ,radio lights
turn on your headlights
Check headlights and dashboard lights first. If no/minimal power for the lights then replace battery.
test
The headlights could have bad bulbs or a bad fuse; the two sets of lights are on different circuits.
Because it is British.
check the fuses - there is no fuse for the headlights.
its probably something to do with the wiring system
Headlights - Cat Power song - was created in 1993.
If it is an aftermarket stereo then i would say that the installer tied into the dashboard illumination or headlight power wire for 12 volt power for the stereo.
If you turn up your interior dashboard light settings, then the radio display will be brighter too. The radio dims to the setting for the dashboard when the lights are turned on.