Which light do you have? Is it a single light, or an assembly with a dome light in the center and map lights on either side?
If it's the latter, unless they discontinued it recently, Ford still has the cover for the light. Also...I have absolutely no complaint about my dome/map light being too dim; could something be up with yours? Maybe the bulb's old/dirty, or maybe the wrong wattage, or maybe there's a wiring issue?
Also, dumb question, but are you sure all the lights are working? Maybe they changed this with later trucks (although I doubt it), or maybe this was limited to XLT models, but my '84 also has a light in each footwell (near the door seam) in addition to the "main" dome lamp.
Just some thoughts...good luck!