Shouldn’t brake lights be of equal brightness? What happens if one is brighter than another?
This can happen if the tail-light or brake light in question was recently repaired or replaced.
WHY ONE SIDE COULD BE BRIGHTER
If the tail lamp or brake light is a good one, there is no reason for it to produce a lower level of illumination.
In fact, even if the unit was not a genuine part, there should not be any difference compared with the original in terms of brightness.
You could check to see if the bulbs used in the brake lights are of the same wattage. If they are, try swopping for a new bulb.
Although unlikely, the dimmer brake light could have a defective bulb, even if it is new.
Most likely, the reason for dim brake lights is a bad earthing point. The interesting aspect of automotive electrics is that the negative terminal is connected directly to the chassis and engine.
Hence, for any component that is powered, the negative can be fed directly from any metal panel near the device. There is therefore no need to run wires all the way from the negative battery-terminal to the individual components.
But the negative wire must be tightly bolted or screwed on to a metal surface that is not insulated by paint.
POSSIBLE PAINT PROBLEM
In your case, it is possible that the bodyshop had sprayed over the original connection point and hence reduced the conductivity of the earthing link.
Removing the connection and scraping off the paint to bare metal, then applying a little grease before re-connecting the wire will most likely solve the problem.
In many brake light assemblies, the negative wire may not even exist as the mounting screws act as the earth link between lamp housing and bodywork.
Paint on the screw sockets will also diminish earthing conductivity.