I’m still trying to remember the name of that film with the armoured car heist.
It might be
Las Vegas, 500 millones (also released as “They came to rob Las Vegas“)
Casino dealer Tony Vincenzo knows how to figure the odds, and he figures the job he’s planned is close to a sure thing. He knows about the route and the security measures taken to protect an armored car hauling cash from Las Vegas to Los Angeles. He knows no one will find the car after he’s detoured it off the road and into a specially constructed subterranean desert vault.
There is another one with Rod Steiger, but I think it is a different one to the movie I was thinking of:
An einem Freitag um halb Zwölf (also released as “The World in my Pocket”)
an enigmatic German woman cons a gang of crooks into robbing an armored car bound for France containing a US Army payroll. The crooks plan to stage a phony car crash to lure the guard out of the truck. They then plan to commandeer the truck, drive it to a carnival ground and open the vault in private.
It turns out that there are heaps of films about aroured car heists.