If the igntion feed has a low voltage, and the alternator is charging and sending a good feed 12v upto 14.4 then the resistance from the ignition feed can cause the bulb to "glow" dim. Caused when ignition circuits from the same supply draw current and the feed to the bulb drops to a lower voltage. Up the ign supply and it'll fix the prob for sure, I have done this on several escorts and it has worked every time. I just soldered a feed from the ign on the barrell (heavy black wire) and ran it to the ignition feed at the back of the intruments multi plug (black wire again i beleive) !!!