I agree, if your bulb is ok & your getting 12v to the light then the fault must lie in the earth (ground) return. Earth faults can be notoriously difficult to find but should be quite simple in this case, good luck.
Fixed. Don't know how, but it is. Took the fender off, inspected everything. Checked where the trailer lights were wired in, un-taped them. They were fine. Re-taped them. Gave up. Put it back together and it worked. I guess it was where I re-taped it even though it looked fine.... any way, thanks for the help.