Smell of burning from under the bonnet of my wife's micra points to a H4 (?) dip/full beam headlight connector.

What's likely to cause this? A short in the connector? I thought the bulb was blown so was going to change it but it sparked back into life and a few local trips has melted the connector.

A dead short should take a fuse out. It could be a poor connection and 'arcing' between the connector terminal and a pin on the bulb.

Dirty or corroded electrical conections create heat which cause more corrosin which cause more heat until it melts the insulation.

Off to a scrappie and buy a decent plug the solder and heat shrink it on to the car.

Poor connection due to corrosion possibly. Increasing the resistance which in turn creates a hot spot / melting / fire

Here's the damage.

I had that happen on a 1992 FIAT Tempra. I thought it would cost pennies to fix, but soon realised I was in for a big bill when I saw the Reg Vardy service guys face turn white when the price came up on the screen. Reg Vardy wanted nearly £500 ( this was in 1998) as it was FIAT's policy to replace the entire wiring loom. The service guy told me to take it to Lucas down the road who replaced the connector for a more palatable £40.

Lol my first car was a micra and I same issue. I just cut the connector off and crimped the cable ends. Then just put them straight on the bulb. Wasn't the best but was only a £300 banger lol

This seems to be an unfortunate weak spot on older Nissan's, as I had this once on my P10 Primera, and twice on my P11 Primera GT. As has been said, get another plug from a good breaker then solder and heatshrink back into the loom. Don't forget to change the bulb, or it will go again.

I've had to disconnect it, tape it up and told the wife to not go far today and not to use her lights and if the smell of burning comes back to pull over and switch off.

Will get it fixed tomorrow.