Actually, there's a scripted mission trigger at 3:50 left on the clock when Kozak joins and starts defusing the bomb. This scripted trigger relies on elimination of all initial SB units which spawn shortly upon entering Barvechos for the first time or after capturing the manager if Barvechos section was restarted upon failure.
If initial SB units don't spawn upon entering Barvechos, clock will reach 0:00, bomb truck will explode and mission will fail. After Kozak joins and mission continues, there are more mission triggers, which need to happen for the mission to progress further. SB rocket helicopter and at least 2 scripted reinforcements (1+ before and 1+ after helicopter) need to spawn to get a call from Bowman and drop-off location for the truck. Sometimes Kozak joins, but never speaks his lines during countdown to detonation, which also leads to mission failure. This is a huge problem in co-op, but it happens every once in awhile in solo sessions too.
Also, random supply convoy going through Barvechos city can interfere will initial SB units kill count during countdown to detonation. Convoy SB units can be overcounted or undercounted. It's a problem either way, since hardcoded triggers rely on a lot of things that can go wrong at any time for any number of reasons. It wasn't always like this though, but the last update broke SNAFU in many ways.
My favorite spot is on top of the building right beside the Truck bomb. Land a Helicopter up top and destroy them all easily. Call in some Rebel support and watch the show. At the back of the building, you can go down the scaffolding and get to the truck. Plus you can take out the few snipers hiding on top of the other buildings as well. Great cover for those pesky fast driving Santa Blancas.