[jsmith948]

Some additional info would help:
1. are the running lights on or off?
2. Is the trailer cord connected to the truck?
3. Are the four-ways flashing on the truck?
4. Is it just the rear side marker lights or are the tail lights flashing?

My first thought would be that you have a short. Also, if you have a bad ground somewhere, the marker lights will dim when the turn signals are activated making it appear that they are blinking.