I just had the same issue with my 49 ford coupe.
First thing would be to make sure your getting the right voltage from off your switch. If that's good this was the fix I did to my gauge lights.
The gauge had these glass caps that are supposed to be a purple colour. Over time, I guess mixed with the temperature of the bulb the glass colour starts to get darker till its almost a black colour. Thus the dim light. The glass cap for the bulb was supposed to work like a black light that would react with the gauge's paint and help it to glow. Of course that would have been when the paint for the gauge and the glass caps were new and would have worked well.
You can take the gauge off (label and unhook everything) and if you look where the opening for the lights are there should be a purple or almost black lens there that you can take out and your lights in your 5 in 1 dash gauge should be bright again.
To take the glass out, (with the whole gauge on my work bench)I took out one of the 4 small gauges for access, where the light goes into the back of the gauge there are these little tabs for the bracket that holds the glass in place, bend the tabs to release the bracket and I used the opening where the gauge I removed was allowing me to be able to pull the bracket and lens out. Then do it over again till they are all removed. (replacing the small gauges before taking a new one out.
You may want to use a coloured bulb as the level of brightness is distracting at night.
This was what I did to fix my issue. I hope this helps