Have you checked wiring? Good ground? Getting full voltage when brakes were applied?
You might need to step up the lumens output to get a brighter output. Most LEDs have don't have a lot of yellow/red light emission and aren't very bright behind a red lens.
That being said, I wasn't happy with the dome light in my 64 so I soldered cheap LED light assemblies together. I used copper house wiring and formed them to fit behind the stock lens. Might be something you could try?
Removed caps from 2 assemblies and formed wire to tuck behind the dome light caps.
https://flic.kr/p/2mWoP2F
Finished, had to slightly bend the assemblies into a curve to fit the stock lens
https://flic.kr/p/2mWvuNu
Great result
https://flic.kr/p/2mWugzB