Electrical conundrum - intermittent sidelights

[lefty67]

Start van in the morning offside front running light and offside rear side light work fine. Go for a short run still working fine. However go for a long run and both go off. Start van a couple of hours later and back on again.
I presume the bulbs and fuses are OK and probably the switch. So would this be a fault in the wiring in getting hot? And how do I go about testing it. Do I need to test it when the lights have failed or can I detect the fault regardless?
Many thanks

[marlinowner]

If you leave the sidelights on when parked, do they stay on? If not what about parking lights ie if you put the indicator on with ignition off do they stay on then?
Lighting switch has separate switches for left and right sidelights, so could be the problem if parking lights are OK.
If parking lights are not working properly either then I would change the fuse as first step.
Fault finding with meter etc will only work when they have failed, but worth wiggling connectors etc to try to generate fault.

[lefty67]

Cheers for the reply.
Stay on as they should when parked and if indicators, headlights on, with ignition on and with engine running
Just happens after a long run. All other lights work except the offside front running light and rear sidelight. Will do what you suggested and will have a wiggle when it does it again.

[lefty67]

Well, it turned out to be the fuse! Not obvious at first as looked ok but after a long run there was a noticeable gap in the metal strip which must have closed again when cooled.
Cheers for the help marlinmower

[marlinowner]

Good result!