I would have given you the same advice asmith just did.
Tracing these kind of problems is trying, but methodically plodding through will solve it.
You have your test lamp exactly in right area. I could not choose a better place to connect than you did
Judging by it's brightness, it looks like your fault would want to draw several amperes. This is when a clamp on DC ammeter would sure come in handy. I'd love to tell you to measure it with your VOM, but you may be facing high current, which is very risky ( to the VOM) if it's not set up exactly right and you don't have the assortment of test jigs.
Whatever it is, it wants a lot of juice.
Try touching the ground cable back onto the battery post to see if you can throw much of an arc. It's a crude ammeter. A nasty arc means whatever it is means business and reconnection of the large negative cables are ill advised until you isolate the cause. A wimpy arc, something like a tail light might do, most likely is some stuck ON load. Still, reconnection will just drain your battery again.
At this point, you may wire your test lamp in and position it to be easily seen. You are gonna be here a while with other things in your hand. If you feel a bit uneasy with the test lamp constantly on, you may attach a small trickle charger across the battery to keep it up. You will be needing your hands for other things.
The VOM will help you confirm a fault, but you gotta find it first. Key off, all doors closed, everything OFF. The only thing ON is the test lamp.
If you had a heavy arc in the above touch test, it may be a fried alternator diode. Disconnect the alternator to see if that clears it.
No? Maybe it's a stuck glow plug relay. Try disconnecting that big red wire going to one of the two large studs on the glow plug relay.
Still on?
The only thing left tied directly to your battery is the starter motor. Not likely, but I'd give it a try. Disconnect it and see.
If the fault is still there, the only circuits remaining are routed through the fuse box. A really nasty fault should have blown a fuse, but obviously it didn't, So it's gonna be some little gizmo that's not turning off.
One by one, pull fuses to see which one will cause the test light to go out
You may take your time with the troubleshooting. Nothing's going anywhere. Just disconnect the test lamp when it's time for a break. You may be be there quite some time verifying circuit after circuit
Hopefully, get a wiring diagram for your truck. First pull the main fuses, one by one, checking each pull to see if it was the one. Keep tracing back until you arrive at the fault.
Anyone else who has experienced the frustration of this kind of wild goose chase is invited to chime in!
I've done this all too often on both my personal van and on my friend's trucks, and it can get bewildering until one collects an assortment of tools, knowledge, and wiring diagrams.