Where to start looking?

[Cajones13]

I appear to have developed a dodgy ground between parking in the sun for about 3 hours and then resuming my journey.
Wagon was fine on way there, but started playing silly buggers for the trip home.
Pressing the brake pedal is causing the "Engine Saver" low coolant alarm to make a beep same as it does when ignition is switched on.
Dash lights and Redarc gauges also dim when braking.
Weirdest part is that when I turn the headlights on, all indications of the dodgy ground disappear and everything behaves normally.
I realise this may be diagnostic, but don't know where it is pointing me :confused:
I installed a good solid body to chassis earth when I first got the wagon in July,
and have just finished cleaning all the battery grounds and installing a new battery to body ground from the Aux battery.
Any advice would be greatly appreciated. Thanks

Chris

[0-TJ-0]

Sounds a bit like a short to me. Haven't done any fuses?

[Reddave]

Not a dud stop/tail light globe? If the filaments are touching might cause the dash/gauge lights to dim?

[Cajones13]

2004 GU.
All my globes seem to be working, think I'm going to have start checking grounds under the dash.
May install a single ground post under there straight from the Aux battery and run all my switches and radios etc off that.

[mudnut]

Check the trailer socket. Add temporary earths from the block to the body to the chassis and see if that helps. Check that rodents haven't chewed any of the wiring loom together.