One thing I discovered on my '02 is that the 7-pin marker parking circuit has it's own fuse and relay (because who know how many lights are on your trailer) but the 4-pin is wired straight into the truck parking light circuit. .
Hmmm, I didn't realize that. I'll have to check it out to see if the '03s are the same... and they probably are. I may have to do some "moving"...

Bill