This is a set of three missions you get when you talk to the Voice after Slim has been arrested. Each mission involves winning over a witness that can help keep Slim from being busted out by his powerful friends, and you can do them in any order, though there's no reward until all three are complete.
The Proverbial Canary
Follow the marker to meet up with Christian Remus, Esquire. Don't tell him that the most proverbial of canaries is probably the one in the coal mine- the one that dies first.
He'll testify if you get his Blanche back. Almost surprisingly, Blanche really is his significant other, not a favorite animal, vehicle, firearm, pinup poster, etc. Follow your marker to the Scrapyard to mount a rescue.
You'll want to bring a sniper rifle with you (or another weapon capable of precise distance kills). If you don't have a good one, there is a Manitou leaning against a rail just inside and left of the main entrance.
Use your rifle to clear the yard. In particular, be sure to pick off enemy snipers in the high towers (these locations can then be looted for more sniper ammo).
When everyone's dead but you, follow your marker to where Blanche is being held. The moment you get close, deputies will start arriving to cut off your escape, so work fast.
There are two health kits in this building if you need them. When ready, step back outside and head into the ambush at the front of the building. You can either take them from the corner, or sprint to the tower and take them out from on high.
Either way there's only a handful of them- about half as many as you had to kill on your way in. When they're all dead, return to Remus to complete the mission.
Instrument of Justice
Talk to a surprisingly pragmatic yet still well-intentioned preacher to get some objectives for busting the Southern Union.
The idea is that you can use the Nightcrawler to take out all your objectives here (all the targets that are up high can be taken out by jumping the right ramp, or destroying the right support beam).
That said, if you get tired of trying to run down the last few targets, there's no penalty for just exiting your car and killing them in the traditional fashion.
You have two rallies to break up. Kill all the marked targets (and as many unmarked targets as you care to), then return to the preacher to complete this mission.
A Good Man
Talk to a vengeful widow for a mission to stop a deputy Tupelo from destroying evidence. The bent cop is at a bar called Landry's. You can drive 'round to a bridge, but it's probably quicker just to swim over.
Something with autofire if useful here- the bar has a lot of hostiles, but they're only too happy to exit in a orderly fashion, straight in to your line of fire.
When you've cleared the exterior, follow your objective to the Employees Only bunker out back.
Corner-kill the holdouts in here, then interrogate Tupelo (it takes only a few bullets to soften him up). Like other Southern Union types, there's no recruit option for this guy- once you get your info he's gone.
The evidence burning site isn't far- lift a car frmo the lot if you can't find yours, and drive over. Keep to the right (southern) side of site, and climb the ladder up to the barn (shoot the guy up top before ascending).
Clear the barn, then ambush the cops out front. When everyone's dead, steal the truck.
What follows is a very short chase. It's hardly even worth going cross country this time- there's a bridge you ned to cross anyway, and the truck itself is tough enough to handle some enemy fire. Note also that pulling into the objective lot ends the automotive pursuit, even if the deputies are right on your tail. However, the cops are still on high alert, so just crouch somewhere in hiding until the alert is canceled, then enter the upper floor of the auto shop to make your call.
This ends the mission.
Completing all three missions will give an objective to return to the Voice. Your conversation with him will launch a final epilogue, granting you a decal set for the Nightcrawler, and tying a bow on the DLC stotyline.