Peculiar Interior Lighting Issue

Question

Recently a friend of mine was driving a VW Fox (not new, I'm not sure what year but it was certainly 5+ years old) home in the dark with the lights on. Strangely, after a minute or two of driving all the interior lights turned off. The headlights, and all exterior lights as far as I can tell, remained on. Turning the lights on and off just turned the working exterior lights on and off (still as intended) but the interior lights on the buttons, dash and alike all remained off. This could be dangerous as it became hard to estimate how fast the car was travelling and other important information like fuel, engine temperature and the infamous check engine light. After another few minutes, the headlights went dim temporarily, the other lights all sprang back in to life started working perfectly! The headlights then resumed their normal brightness and they returned home with a perfectly functioning car.

What could be the cause of this? Will it happen again if nothing is changed?

My current thoughts are:

• Probably not alternator problems as headlights are still working and bright as always
• Could be loose wiring or a short circuit
• Some sort of electrical fault (though no EPC or check engine light shown)
• May have poor battery condition so the car is radically trying to reduce consumption so as not to require a jump start on the next drive (though I doubt this somewhat)

Answer 1

There's one obvious device that controls all the interior lights, and that's the headlight switch/dimmer. On some VW cars, this is one large switch, and on others there is a separate dimmer. The dimmer is normally a horizontally mounted roller that you can roll up or down to dim the interior lights, or when rolled all the way to the "up" position it will click and turn on all interior lights at full brightness.

The next time this happens, try to roll the dimmer up and down to see if it makes a difference. Even without seeing the problem, you can try to play with it to see if there is a "bad" spot where the lights flicker or go out.

Of course before power even gets to the dimmer switch the headlight switch has to turn it on, so that is suspect as well.

And for completeness, I agree with @rpmerf - it can't be a fuse. It could be a short in the wiring, but I would fully diagnose the headlight and dimmer switch before even considering the wiring.

Answer 2

Since the lights came back on, this is not a fuse issue. If there was a short, it should had blown the fuse. Did the radio remain on?

There is likely a loose wire somewhere, but unfortunately it is almost impossible to test while everything is working unless you can recreate the problem.

A couple things I would try:
Get a schematic, look for any junctions, plugs, fuses, switches, etc. on that circuit
shake wires on the effected circuit to see if they cause the lights to go out
Check grounds - this will typically be a strap cable, or a black wire attached to the body
The headlight switch is suspect, given that the entire circuit went dead. If you can test while it is not working, test at the output of the headlight switch.