Taillight problems

Tail lights do not work. Checked bulbs, checked fuses, when the bulbs are in the brake light on back of cab comes on. When I take the tail light bulbs out of both sides the brake light goes out on the bake of cab it works normal. Bulbs in light on the back of cab is on, bulbs out the light works normal. When the bulbs are in turn signal on either side both lights blink including the brake light on back of cab. I have three maker lights mounted under tailgate the lights are on when bulbs are out, put bulbs in the three marker lights go out. Checked bulbs in front both sides and they look good. The lights work fine in the front and dash lights work fine. Any suggestions? John.