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Or not.
When I turn the headlights on on the van (Bongo), they're ok for a second or so, then dim to a very faint glow, on both sides. High beam is fine though. Anyone got any ideas?
poor grounding
As Ibis says, poor grounding/earth would be my first suggestion.
Sounds like an earth fault, check the bulb holders for contact and give them a clean with some wet and dry.
Take the bulbs and holders out of the headlamp, clean the contacts and if any corrosion, use some fine sandpaper to remove it. Check any earth points in the engine bay and again clean any corrosion off them.
Are they HIDs or anything fancy?
Had this with the rear lights on the 306. Cleaned the contacts and they were better. Sometimes it's easier to put in an extra earth