Intermittently, dash lights and rear running lights go out when headlights turned on. Flipping back and forth from parking lights to head lights it usually works. Also note a "trailer" image when the turn signal is turned on. Is this normal?

 

If the trailer light comes on when using the turn signal, that normal means it has been fitted with the heavy duty relay required for fitting the trailer wiring.

This might also be why you are having a light problem, in that the disco may have had fitted a trailer towing socket and tow bar, and has either a) been removed badly, and thus causes intermittent problems with shorting cables.. (i.e the cable was just cut at the back)
or b) is still there but has a lot of dirt or is allowing water ingress and thus causing a short.

 

I just went though the same thing with my rover. It is your head light switch. It is worn out, it is easy to replace, but rather expensive. I tried a used one but that failed after a couple of months. The switch is just a bad design. go to www.britishpacific.com
the switch is $135.00.

by the way you did not get the whole story on that trailer light. all discoverys have it and it is a bulb check for a trailer. It should flash once and than stay out. If yoy have a bulb out or a short in your traler wiring (when the trialer is connected) it will stay on.

 

Only tests the indicators (turn signals) not the brake lights (stop lights) or the general lighting.