Trailer side lights not working ?

[bigdog]

My boat trailer 'side' lights and number plate light are not working ?
(Running lights)
The directional and stop lights are working ok, no problems !
I did notice however, that these lights do not stay on 'all the time' and I believe they should when trailering boat at night.

My trailer wire connection to truck is your typical 4 wire plug.
(green, yellow, black, white)

Thoughts as to why side lights are not working ?

Thanks !