[jimjaz]

JM, Welcome! A big block V6 down under? I am thinking that must be a rarity where you are. The short version; get a new flasher like the one you have in it.
The more long-winded version;
Are all your lights working?- brake lights are dim and light up bright when you push the brake pedal with the headlight switch pulled out for night driving? And then 10 minutes or so later everything works the same except your turn signals front and rear when you flip the turn signal switch left or right? Original wiring or any new "additions" that you recently added?
In my experience, a bad turn signal switch will just plain wind up being bad for whatever reason- it won't work for roughly 10 minutes and then act differently. That brings me to the turn signal can (flasher) that is literally plugged into your fuse box up under the firewall on the driver side. When your turn signals stop "flashing" and you have this problem, do the left or right bulbs light up brighter front and rear, or do they stay dim? Feel this flasher can- is it hot?, making any noise (tick-tock)? Pull it out, give it a few minutes, then plug it back in. Is it working again after giving it some time to cool down? Might be the turn signal flasher- most times these will last forever, and sometimes they can be persnickety and act up. This problem from your description sounds heat-related electrically, and you do not have to have your engine running to diagnose this. Pull your headlamp switch out from off to the parking/running light position without the headlights and see if you get the same symptoms. Or just try a new flasher can- they are pretty inexpensive, I'm thinking under $5.00 US. Don't know if these flashers are specific in nature but I would get one that fits with the overall application and year of the truck.
I am no expert when it comes to automotive wiring, but I do know that electrical contacts do develop a "native oxide" over long periods of time, and this is more common around high humidity/water areas. Salt water is corrosive and accelerates this degradation. It is possible that pulling the turn signal canister (flasher) and plugging it back in may actually solve your problem without having to do anything else. Do this 4 or 5 times and try it again. As long as you are down around the fuse box, pull out your fuses one by one and check the copper contacts of the fuse box and fuse ends- they should all be clean and shiny. You can clean this up with a wire toothbrush.
If you did something to the truck by adding whatever and THEN started having this problem, undo what you you did and try it again.
Try some things and let us know what you come up with.