Over the past few months I've been getting an intermittent error on the dash after about 15 minutes of driving. It's either a general bulb failure or states registration plate bulb failure. It then goes after 30 secs and by the time I've pulled over it looks fine.

I've replaced the lenses/bulb holders and bulbs for the registration plate light and cleaned the contacts but it still occurs. Could it be grounding issue? How do you remove the boot trim (saloon) so I can chase the wires?