Actually, it can still be a bad ground. Check all bulbs and all
connections to the trailer and clean the corrosion under them. I had a
very similar problem and made similar inncorrect assumpions regarding
no lights would work if the ground was bad. But an old salt convinced
me to go over all the connections to the trailer and it fixed my
lights. I knew it was in my trailer and not my truck as my truck
worked fine with another trailer. So don't go messing with your truck
until you try this.

Jerriko

I haven't even gotten to the trailer yet.. Im still working with a
tester that fits right into my 4 prong connector on my truck. If the
tail lights are on, then neither of the turn signals will light,
whether i press the brake or turn the turn signal on.. nothing. I
assume that my tester works just fine.

But heres the weird part.. when I plug the tester DIRECTLY into the
taillight converter (which has absolutely no indication of any ground
at all.) The tester still lights (well the parts that light anyway..)
But If I plug the tester into a 4 prong 'extension cable' which has a
ring terminal where the ground should go, nothing at all happens,
unless I ground it. So I have some sort of mystery ground.. heh.

Anyone ever hear of a test light doing anything like that, and NOT the
lights on the trailer?