How do you think your grounds are good? Most Broncos suffer from lack-o-grounds, so it's still a possibility.
Especially after your description of the flickering.
Some things that can cause the flickering:
1. Loose sockets. At this age, they're probably not all getting a good connection to the bulb.
2. Lousy grounds. All of them ground through the little steel tabs around the circumference of the socket, to the cluster housing. Strike two there!
3. Loose wire connections somewhere. Again, usually at the sockets.
Oh, and did I mention a lousy ground scheme?
First, the truck is however old it is (Add what year it is to your signature area so we can see. It can make the difference sometimes) and all the bonding between metal components is sketchy at best by now. And the dash is a classic EB bad connection waiting to happen.
The lights ground through the cluster. The cluster grounds through the screws to the dash. The dash grounds through the bolts to the body. And by now they're ALL rusty.
The body is supposed to ground directly the the battery. It often doesn't and then you're stuck trying to pass current through your engine mounts and radiator. Not optimal by any means.
Even when there is a good ground wire from the battery to the body, it's usually to the wheel housing. Which might no longer be well connected to the rest of the body, depending on rust levels again.
So, when you say the grounds are good, did you verify all of the above?
If so, you're on your own buddy!
Just kidding. You're never on your own here. Just keep digging. I forgot to mention the connection at the switch itself. Viper mentioned the rusty fuse contacts, but sometimes rust AND heat will damage/loosen the contacts right at the switch connector itself.
Good luck.
Paul
