When the plague first hit and the powers that be understood that light was a weapon they created a sort of firewall using the bridge. Unfortunately they lost power at a critical point and the bridge was over-run, which is why they ended up blowing it in two sections.
Here is the thing - the Tower's Engineer thinks that now, with the power restored, there may be some value in the bones of that system!
What he wants you to do is - at night - go to the bridge and recover any of the bulbs in the special lights that are still working (meaning not burned out) and bring them back. The problem with that is that as you remove the working lights, the bridge will go dark - and the infected will come in. Making it a very dangerous place very quickly!
He is not sure if this is as good an idea as he originally thought - but you flag the quest anyway...
Wait until dark then make your to the bridge and climb to the top deck - cross to the other side and climb the outside of the opposite support and then zipline across - take out the infected and continue up until you reach the area with the lights.
Collect all of the working ones then climb the other tower to get to the next area and collect the rest of the lights. Now simply head for the Safe Zone behind the door, sleep till day and then dive off and return to the Tower to collect your reward!