on my 2001 dodge dakota left blinker blinks fast but when my pull behind trailer is hookup to the wire harness it blinks normal what is causing this

The blinker bulb on the car is out or shorting, check all the blinkers on the left side of the truck, don't forget if it has side blinkers ck those too.

From what Ive seen usually a bad ground wire connection either on the truck or the trailer. Check make sure the blinkers are working correctly on the trailer. If not check trailer ground first

i have and replace both in the front work fine for a couple of days then they started to blink fast again both light up but only one side at a time in the front rotating between the two

So if you put new bulbs in the frt they work properly 4 a couple days, is that correct?

yes then it started to blink fast again check all the bulbs again they all light up the only time it doesnt blink fast is when i have my trailer hooked up im lost on this one

I would try replacing your flasher rely it maybe shorting out internally

I don't haul a trailer so am unsure how the lights are wired, does the trailer wiring have it's own blinker relay? My thoughts are that the turn signal relay is the problem but only if the trailer has it's own in the harness. This didn't start after hooking up the trailer lights did it?

if your using the trailer hook up that came with your dakota then check the ground,if they flash fast when no trailer is hooked up but flash correctly when one is,then there is a ground problem and the stock hookups are known for this problem.if it flashes normal until the trailer is hooked up then the problem is in the trailer,you may need to reground the trailer lights,