Found rotted trim pieces

Not sure if this is the right forum, but it does say "waterproofing".

Anyway, I found some rotted trim around my garage door today. The trim piece that is at the top of the door (parallel to the ground) and the side piece right next to it are rotted. I noticed paint peeling over the winter (which I just primed and painted the pieces 2 years ago ) and today few black bumps. I pushed on the area and my thumb could have gone through the trim board if I pushed hard enough. I am going to take down the rotted board and replace with composite fiberboard (which won't rot), but I am concerned about where the leak is coming from. The only place I can think of is just above the garage door, we have a trim piece with a little keystone type piece in the middle. This whole trim piece sticks about 1 inch from the face of the house and I think the vinly siding slides right behind it. Is it possible that the flashing here is missing or no good? It is a townhouse, and the front face of the house above the garage door is flat and 4 stories tall, so I *hope* that the gutter above isn't leaking behind the siding and draining down the face of the house until it hits this trim piece and stops to rot it out.

Any thoughts on what/where to look for the source of the leak? I can fix the boards, but I am concerned about the bigger problem of why it was rotting.

Thanks,
Neil

 

Is there any sign of water damage above the door on the inside of the garage? If you can't see anything from the floor, go on the ladder & feel for dampness. Try to find the highest damp spot. That's where the leak is.

 

I looked from the floor and didn't see any obvious sign of water damage (no discoloration of the drywall). I will be getting out the ladder to get closer and probably cut an inspection hole to look at the header boards that the trim piece would be nailed to to determine if any of that rotted also. I plan to insulate the wall above the garage door, so I want to find and fix the leak for that reason also.

Thanks,
Neil