Recently Codemasters have removed GFWL and had to release a new game on steam in order to get rid of it.
After they did, the original DiRT 3 was removed from steam and currently is hidden under a different community hub.
Which means all guides are worth nothing anymore.

Anyways, Yes it is too late since 2011 but its always nice to help new players.. right?
I never played DiRT 3, i never bothered getting it because people told me its not worth it with GFWL so i am indeed a newbie.
However, i did complete 100% of the game and the challenges so i marked the locations of the boxes in red because they won't appear after you picked them up already.

Package 1 out of 9:

Can easily be found under the big dust ramp.

Package 2 out of 9:

Under the truck trailer

Package 3 out of 9:

Behind the ramp on the other side, inside the blue container.

Package 4 out of 9:

Drive up the ramp and it should be on the edge of the container.

Package 5 out of 9:

Can be found where most containers are located between the boxes.

Package 6 out of 9:

At the end of the map at the back between the boxes.

Package 7 out of 9:

Jump from the dust ramp and pick it up in the air.

Package 8 out of 9:

Near the construction site, before you take the jump you can pick up a package.

Package 9 out of 9:

After you picked up Package 8 you can turn right and move towards the top of the truck trailers and pick it up between the boxes.

Package 1 out of 7:

Near the river under the truck trailer.

Package 2 out of 7:

In the warehouse between the boxes.

Package 3 out of 7:

Under the ramp behind the warehouse.

Package 4 out of 7:

Behind the Crane and the breakable wooden fence.

Package 5 out of 7:

Blue containers near the Cranes and small wooden bridge.

Package 6 out of 7:

Near the small ramp and crane.

Package 7 out of 7:

After you picked up package 6 go to the ramp and drive til the end and the last package is located inside the blue container.

Package 1 out of 8:

Under the bridge near the river.

Package 2 out of 8:

Under the underground entrance road.

Package 3 out of 8:

Before entering the construction site there's a little room which can be found to the closest cranes.

Package 4 out of 8:

Enter the construction site, this should be around the middle on the first floor.
Jump on the pipe and pick it up.

Package 5 out of 8:

Go on top floor of the construction site and pick up the package in mid air jump.

Package 6 out of 8:

There's a dark room near the main entrance of the construction site and the package can be found behind that wall.

Package 7 out of 8:

Inside the pipes, simple drive upside down and pick it up.

Package 8 out of 8:

Do a barrel roll and pick up the package in the air.

This is my first and hopefully not last DiRT 3 guide.
I didn't put too much work into this guide than my usual popular ones in other games but its still better than nothing.
Enjoy picking up all your packages :)

Q: Why did you not include the driving objectives?
A: You can simply read the "Missions" at pause or pregame menu and know what to do.

Q: Why are you making this old guide?
A: Game was recently re-released on steam store and had a content wipe.

Q: What's this car?
A: Mitsubishi Evolution X - Yokohama edition


Credits to SuperGT for the packages order.

Pretty much it, enjoy :)