My HJ75 landcruicer ute has an electical issue. rear indicators, brake lights and reverse lights work fine until the parking lights or headlights are turned on. The rear left hand lights are then working dimly and all flash dimly when the left hand indicator is turned on.

Almost certainly a bad earth in your left hand rear light somewhere.
Clean the earths up REALLY well, ( like silver shiny metal ) and put a new end on the wire to be sure.
You can test it by grounding one end of an extra wire on a really good contact point on the chassis and getting someone to hold the other end on the common earth contact for your rear left blinker, stop light, reverse light and tail light while you try it again.

Definitely not getting a proper earth somewhere.

I have also seen a whitish corrosion on the bottom of the bulb itself, and in it's socket, cause the exact same symptoms.

If there is evidence of water in the taillight lens, go for the bulbs and their sockets first. Clean, clean, clean.