Common reasons for dim lights are, Failing alternator that isn't keeping the battery charged, Failing battery that is pulling the voltage down. Yellowed plastic that is blocking the light. Old bulbs, even the best bulbs will lose some lumens over time due to deposits building up inside the bulb case.
The biggest issue I see however is corrosion in the connections causing reduced voltage at the bulbs themselves. The corrosion acts as a resistance and instead of full battery voltage at the lights you get a reduced amount, even a 1/2 volt drop can cause a light to be noticeably dimmer. This voltage drop is why manufacturers started adding relays to the system. The relay allows for a shorter run from battery to lamp which reduces the voltage lost in the wiring and switches. For the cars with corrosion, cleaning the corrosion off the contacts and grounds combined with new bulbs and adding dielectric grease to the various connections to slow new corrosion can make a dramatic difference.
Friday, April 20th, 2018 AT 8:44 PM