HB - suggest yopu go to Harbor Freight on E Ridge & buy a 12v screwdriver type test light. (roughly $5) Best investment for trailer lights I ever made.
Also, with someone working the lights for you, check which filaments light up. Brightest ones should be the brakes.
I couldn't get my brake lights to come on couple weekends ago, but others worked. I found some corrosion inside the socket. Cleaned it & greased the bulb contacts ...Whoa-la , let there be light!
Tom B.
(LongLine)