Hi there,

If you had a volt meter you could easily check for 12 volts.at the bulbs.

Maybe look towards bad earths around the lights
They usually us the chassis as the return wire as it was to the battery.
The connection to the Ground/Chassis can get corroded with age.

The dim one is like have a resistance in series to earth so the
bulb wont light fully.

If the bulb is a single connection then they are using the outside metal
of the bulb to make the connection to the Grounded side iof the bulb holder.
Clean both.

A cheap volt meter here in the UK can be purchased at Maplins
for circa £3 and its a good investment.

Johnny UK