You can use a trigger and synchro it to the waypoint using whatever condition you want.
The trigger would read something like:
activation:east/present repeatedly?
condition: truck_name in thisList
onactivation: none or call a script
This assumes you are checking for a particular truck, if its any east unit then leave the condition field as "this".
A second option is to set the conditon field of the ambush wpt to something like ambush == 1, then use a trigger with the confitions above and the onactivation field set to ambush = 1.
Both essentially read as once an east unit is inside the trigger area, move to this waypoint using whatever conditions you set for that waypoint.
SB...