Where were the shootings done from in Jack Reacher?

The parking garage

It's clear from the novel, and to my mind the movie, that the shots were made from the parking garage.

The novel opens (as does the movie) with an unidentified man parking up.

First Street opened out and became slightly shabby again. There were bars and dollar stores. Then a parking garage on the left. Then yet more construction, where the parking garage was being extended. Then further ahead the street was blocked by a low wall. Behind it was a windy pedestrian plaza with an ornamental pool and a fountain. On the plaza’s left, the old city library. On its right, a new office building. Behind it, a black glass tower. First Street turned an abrupt right angle in front of the plaza’s boundary wall and ran away west, past untidy rear entrances and loading docks and then on under the raised state highway.

But the man in the minivan slowed before he hit the turn in front of the plaza and made a left and entered the parking garage. He drove straight up the ramp.

There was no barrier, because each space had its own parking meter. Therefore there was no cashier, no witness, no ticket, no paper trail. The man in the minivan knew all that. He wound round the ramps to the second level and headed for the far back corner of the structure. Left the van idling in the aisle for a moment and slipped out of the seat and moved an orange traffic cone from the space he wanted. It was the last one in the old part of the building, right next to where the new part was being added on.

^{One Shot - Lee Child}

The bridge is alluded to because, that's where the shots should have come from IF the shooter had been a complete professional and was interested in not getting caught.

In the novel, the "bridge" is an elevated section of highway which is perhaps why you could not find the reference.

There was no barrier, because each space had its own parking meter. Therefore there was no cashier, no witness, no ticket, no paper trail. The man in the minivan knew all that. He wound round the ramps to the second level and headed for the far back corner of the structure. Left the van idling in the aisle for a moment and slipped out of the seat and moved an orange traffic cone from the space he wanted. It was the last one in the old part of the building, right next to where the new part was being added on.