If you have a voltmeter, check the input and output voltage for the brake lights at the converter box. If there is more than 0.5 volts drop across the box, you may want to look at replacing the converter.

A good converter will have about a 0.1 volt drop and a poor one about a 1.0 volt drop.

All converters are not the same. The dealers and places like U-Haul usually have very good ones. Places like Walmart usually have good to poor ones. Depends on who was offering the best price.

Anouther problem is actual connection to the cars wiring. If you used those little blue plastic splice things, the connection is probably corroded.

By the way I hate trailers, trailer lights, and the wiring for trailer lights on my car. The people who build those things are always trying to save $0.02 which ends up costing me $10.00 2 years later.

We all know the deal, you discover the U bolts on your galvenized trailer are zinc plated and when one breaks you have to use your mainsheet and a roll of duct tape to hold the trailer togather to get home.