is there a constant 12 volt source in the rear of the truck, I need to wire a courtesy light in the fibreglass cap and possibly a 12 v lead to my trailer

 

Your profile says Dakota but your in the ram section so I'm not sure which you are asking for but either way, take a volt meter or test light and try to 7 pin trailer harness plug

 

Consider running a fused/breaker circuit directly off of your battery if you don't already have a power wire for a trailer.
Steve

 

If your truck comes with a full factory 7 pin plug for towing then yes, one of the pins on a 7 pin is constant 12v, if your trailer uses flat 4 id suggest switching to a blade 7.

 

my Dakota came with a pin, I have changed to a 7 pin and added the brake controller etc etc

I guess the best solution is a new fused lead from the battery or an unused source in the fuse box

I was hoping the main harness may have an unused wire in it!!

 

Not unless your truck came factory with a 7 pin, the more common 4 and 5 pin headers have no constant 12v so neither does the harness. Like you said I would run a decent gauge line from your PDC off a fuse/breaker and wire it into the correct location on a 7 blade, that way you are future proofed for any future towing that may use a 7 blade like most campers do.

 

im already towing a 3000 lb travel trailer!! and have added the 7 pin connector but without the constant 12 volt, ive already run wiring for the brakes

the 12 v lead will the trailer battery charged if we leave a light on in the trailer

 

If you are running a dedicated line to the trailer, AND you have a battery on the trailer just be sure to use a battery isolation circuit, that way your truck can charge both batteries but leaving a light on in the trailer will not drain the trucks battery.