Why did the gang fail to check the trunk of Walter's car in the final episode of Breaking Bad?
They should have been cautious, knowing Walter White is very smart. They only checked what's in his clothes.
Because Kenny (who met him at the gate) was lazy and stupid and more interested in mouthing off about his knowledge of the car (which he might have been intending to soon own) than checking the guy they were about to casually murder.
..knowing Walter White is very smart
I think Jack and Todd knew that, but it is arguable that Kenny felt he was pretty stupid for walking right into 'their trap' for him, expecting to get power, respect or money for a new meth recipe they didn't care about in the slightest.
This was due to two things they felt Walt did not know:
Kenny had no way of knowing that Walter knew Jesse was still alive, and Walter neither knew nor cared that the 'running short of methylamine' argument that got him in the door was irrelevant given a continued supply. As long as it got Walter and the car into the right position to attack the gang, he was happy.
Walt: You're running low on methylamine, what happens then?
Jack: No, we'll get more. That Quayle woman's got her own set-up. The heat's off her. She can break loose a barrel every now and again. It ain't broke, so why fix it?
Because, really, who builds a remote-controlled machine gun turret and installs it in the trunk of their car? There have been countless "meets" between good guy and bad guy in cinema and TV, usually in a remote location where cars are driven. Have you ever, once, seen one of those guys check a trunk? It was an original idea by the writers, so just give them kudos for thinking of it.
Think about it. Why are the trunks of cars checked at the gate anyway?
Maybe because you might have something suspicious in the trunk which you might take out secretly, once you are through the gate and inside the parking where no one is watching you. But this was clearly not the case as Walt was supposed to be escorted right away by Jack's men into the room. I mean according to the Nazis' plan, Walt was not to be left alone once he arrived at the facility.
Maybe someone can store explosives in the trunk of the car which they park and explode. Usually a suicide mission. This was not the case again. The Nazis could not have thought that Walt was on something like a suicide mission. They really thought, Walt was there for talking business.
Now regarding this whole situation, in my opinion the Nazis were quite smug, complacent and over confident at that moment. May be they knew Walt was a dangerous man but they heavily underestimated his last move. They were clearly outsmarted by Walt. They did not have the slightest imagination that Walt was there to kill them all. There could be two reasons for this: