hmmm... this could be another of those version discrepancies - setting the pbr combat mode to stealth should do it.
if that doesn't work, i would suggest getting rid of the driver with a setpos, and then moving him back in when needed. drastic, but it would work.
pbr_nameD setpos getpos elsewhere_logic
pbr_nameD unassignvehicle pbr_name
to get the driver out
pbr_nameD moveindriver pbr_name
pbr_nameD assignasdriver Pbr_name
to get him back in. untested, but should be along those lines.