I found a Combination Turn Signal and Side Marker Light from Levine Auto and Truck Parts that I installed on my trailer. It uses a bright 3157 bulb. I tried using LED lights but they were almost impossible to see in daylight. I connected the Marker light to tail light wire and the turn signals to each right and left turn signal wire.
Easy to install and they are very bright in daylight.
Yes, I mounted them left/right side in the middle of my trailer. Mine is a travel trailer with about 12 inches of metal skirting above the tires. Mounted them to that and the connected the marker light to the wire running to the tail lights in the rear. For the turn signals I connected the wire to the left/right brake light wire. Both will come on when you put on the brakes if not using the turn signals. I ran the ground wire to the the frame ground connector. They work great and are very visible in bright sun. The LED's I bought were OK for night time, not visible in daylight.