Probably because the dimmer switch is turned off. Turn the dimmer switch up and see if the dashlights and radio illumination lamps come on.
Probably a loose wire or a short in the wiring that powers the dashboard lights.
Also check the fuses and the wiring to and from them.
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.
Most often the tail lights and dash lights share a single feed and fuse. Assuming you have checked the fuses (probably marked as tail or courtesy) then I would next check the head light switch. If you have good fuse and the head light gives a voltage out while in the 'parking light' position (usually between off and headlights) then you need to start chasing wiring. If you need more, post another question, and I'll see if I can walk you further down the path. I had this exact same problem and found out that the headlight switch was just making enough contact with the plug for the headlights to work. We plugged that in and the tail lights and dashboard lights started working also.
pull out on the end of the stalk that has your headlights on it
check the fuse that's if the bulb is not burnt out.
At night with your headlights on you can only see just so far down the road. Overdriving your lights is driving so fast as to not be able to react to something that is beyond the beam of your lights.
turn on your headlights
The headlights could have bad bulbs or a bad fuse; the two sets of lights are on different circuits.
The "trip odometer" on the dashboard doubles as a dimmer. So, the solution to no lights inside the Toyota Sienna is to turn the trip odometer up to turn the lights back on.
check the fuses - there is no fuse for the headlights.
Turn the stem for the odometer reset
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.
Turn on headlights/parking lights and adjust dimmer control
You turn the trip odometer button on the dash.
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.
Because it is British.
its probably something to do with the wiring system
Check headlights and dashboard lights first. If no/minimal power for the lights then replace battery.