There are 2 ways that I know of.
1) Backflip off the fence (where the door would be opened if sector 3 opened at the spy spawn) then goto the back of the pole (left arm up) and press left and b at the same time. Walk very slowly in the wall, dont go in the building or your stuck. Then climb up the pole, then walk back out the hole on the right side slowly. Once your out far enough you should be able to stand up. Then jump on the building and then the garage door, and then off.
2) Inside the building, you go outside the door of the mercs spawn where the ammo is. Stand infront of the ammo box and backflip on the wall right infront of you. (It is very close and you may have to try it a few times) Then you have to get back to the Spy spawn w/o hanging or climbing on anything! I run through the middle section on top, then jump on the railing, then the box, then the lifter, then the conveyer belt. Walk to the spy spawn then jump down.
Then after choosing the way you wanted to, you climb up the front of the pole, w/ left arm up press left and b.
If done right you should be in the Merc's Spawn.