The two protagonists are transporting a MacGuffin in an inconspicous truck which approximately looked like this: The truck was internally armored and bulletproof, it had a turbo-/supercharger which could be activated by a security switch and allowed very fast driving (estimate: 150 mph/ 250 km/h) and under the round hood on the top of the driver cabin was a hidden maneuverable gun. The drivers are in steady contact with the headquarters over radio. This is also the part I am 100% positive that it is correct. Unfortunately despite being standing out, I was unable to find any flick having a fast truck with a gun on the top.
Whatever the MacGuffin is (I do not remember), it is not money. I am not sure the truck was escorted, but I think so. The villains are portrayed as terrorists in this case; indicators were that the whole operation was not a normal transport (else they simply could have used a normal APC or armored transport for this case), the villains had access to confidential information, they acted ruthlessly and intelligently (kidnapping a family and risking their lives is something which criminals are wary to do, robbers who do this are mostly not exceedingly bright) and lastly they fought to the death (criminals will normally give up after they recognize they are outclassed). Their actions had the whiff of the True Believer. There were 6-12 villains.
The group of terrorists including at least one woman (rare for the 80s, possibly even the leader of the group) kidnap a family and place them on the street so the truck must stop (eventually taking out the escort?) in a dead spot without radio contact. I think it was near an electricity station, the location was, if I must guess, in the middle/north USA (Nebraska et al). Once stopped, the terrorists are jumping the truck from the sides and damage it with explosives so badly that the truck systems are shut down and the protagonists are helpless. The gun can still fire, but the mounting is permanently damaged and the gun cannot be aimed anymore. Because they are in a dead spot, headquarters has no clue that the situation is compromised.
After having the MacGuffin, the terrorists flee in cars and at least one pickup. But our protagonists are able to get the truck running again and even better, to activate the switch so they can outrun the terrorists. Fight, win, terrorists dead, family safe, happy end.
The whole setting has a TV movie touch (but a better one), I am quite sure it isn't a movie production. Nate Eldredge asked if it could be one of the crimefighter series; while I wouldn't bet my life against it, it does not feel right. My fallible memory tells me the movie was standard length (~90 min), the pacing and the background music wasn't right, it wasn't a pseudodoc with a dramatic voice from the off and if the Hoff would have appeared, I would surely remember that, you know, German .... :o)