I have a 1996 Plus 4 that has wing mounted side lights as well as side lights bulbs within the headlight units. The light switch is a rocker switch that operates firstly the side lights followed by the headlights.
These operate normally until I switch on the ignition when in side light only mode, the dip beam filament glows on half power.
Using a voltmeter I get a reading of 5.9v at the bulb connector.
I had seen this before and assumed that it was part of the original wiring set up without the wing mounted side lights. The side light wiring within the headlight unit is not connected under the bonnet on both sides.
Looking at the various wiring diagrams available on Gomog I note the reference to a dip dim module. Not sure if this is included on a 1996 version.
Can anyone advise if this a normal configuration or maybe I have a cross circuit somewhere that's producing the low voltage feed to the headlights.

It is almost certainly the dim-dip unit working. This connects the dip beam filaments of both headlights in series when you have the side lights on and also the ignition on. It was a legal requirement in the UK for a number of years in the 90's until the EEC decided it was illegal of the UK to make it a legal requirement, if you follow me.

Normal.
But, dim-dip units are notoriously unreliable. Suggest removal if yours gives trouble. Whether you connect the marker lamps in the headlights at that stage is up to you...

Thanks for the responses. At least there's nothing actually wrong with the wiring.
There seems to be a large voltage drop on switching the side lights on with the engine running. This is why I was wondering about a problem on the dimmed headlights.
Maybe I'll disconnect the dimmer unit, if I can find it, and see what difference it makes to the voltage drop.

I had a similar problem (89 plus four) Kevin Vernon removed the relay and that was the end of the problem.

The dim-dip unit is a black plastic 3cm cube with about six or seven different coloured wires sprouting out of it, hidden behind the dash high up in front of the steering wheel area.
If you remove it, you need to connect together the two blue/white wires that were attached to it, also the black/orange to the black (earth); insulate the others and just leave them.

Thanks,
I found the unit although the wiring between the unit and the connector are different to the wiring diagram, no surprise there then.
I just unplugged the unit and I no longer have the dimmed lights or the large voltage drop shown on the voltmeter.
I've done a check of all lights and other electrical units and everything is working normally.
There are only 4 wires on the unit itself, white, black, red and purple. These don't match anything in the wiring diagram so I guess it is component from another vehicle or supplier.
Looking at the diagram the lights are on a duplicate feed so they don't rely on the feed coming via the dim dip module.

Glad you have sorted it out. It appears that you have a later type of dim-dip to mine (1993) - I see from Rutter's parts website that MMC changed them in about 1996-1997.