Dash light sometimes fail to come on at night

This is a different model in the States. If this has a display for the Navigation there are controls to set for brightness and auto adjustment for day and night. These controls are reached within the programming of the Navigation screen and independent of the dash lights switch.

You are probably zoned out of making adjustments when the screen is dark. So you will need to tinker with this when you can see the screen better.

On our vehicles the headlight dimmer switch only controls the dash lights. Could be you had this vehicle serviced. Sometimes when an OIL CHANGE is done the service technician wants to reset the oil interval and goofs up some other settings in the process. Think about when the problem started and if someone else had their hands on your vehicle.

is this car the all alluminium xk model ? if so it sounds like a bad earth the earth studs are all bonded to the body and are prone to detatching from the body and causing all strange probs. the favorite ones are both front behind the wheel arch liners, if its the older xk still sounds like a bad earth poss behind the headlamps. the only other probs i had was wires coming detatched/poor connection from the column switch, hope this helps

It will be really hard to find the problem is a bad ground ...batterie ground to the body...under dash ground

do the rest of the lights come on, like heater controls and radio display lights and you say sometimes, do they work at times and not? you listed this as a 1999 Chrysler LHS is this the correct model?

The same thing happens on my 2003 Impala. I found that my dimmer switch has an intermittent failure where sometimes the dash lights go out even with the headlights on.
If I wiggle the dimmer just right, the dash, radio, steering wheel, door locks, and every other light comes on (when the headlights are on). My fix was to mount my Garmin on the steering column in front of the speedometer so when the lights go out I know how fast I'm going. A replacement dimmer (or bypassing the current one and adding a resistor) should take care of it.

Failed Battery is where I would start my testing

This seems to be a common problem with the Impalas. Most find that replacing the headlamp switch fixes the problem. It seems the dimmer inside the switch tends to fail causing the dropout when in night lights mode.

You may have air in the system, or possibly a failing thermostat, or low coolant.

the occasional not working leads me to believe it's loose wiring or a short in the wire somewhere