there is a red paint htey make for anti corrosion, you scrub that battery down and put the paint on and it helps a lot. it may mean your battery is going, but im not sure.
as far as the dim light...have you replaced the bulbs? tried switching sides? it may be that theyre different brands.
what is the non-running voltage out of oyur battery?
running voltage?
should be 10.2-11.5 and 13-14.5 respectively i believe.