Tim
Overrun is a MPh activity
Vehicle declares Overrun
MP are spent which can be used by defensive fire OUTSIDE the hex or from INSIDE using IFT/IIFT (agains any CE crew etc) a gun or LATW
Assuming vehicle survived or wasn't immobilised (if it didn't Overrun still happens with 1/2 FP)
then the vehicle completes the overrun
AFTER the overrun has happened the defending infantry that have survived can then use close combat reaction fire per D7 .21
(it can cause do the other things IFT gun or light anti-tank weapon as well. but if you had these things in the 1st place you probably should use them before the overrun not after it !!)
This just happens to use the close combat rules - but it is done during the movement phase
In certain circumstances an ambush can occur this is a special kind of ambush called street fighting. This is found on D7 .211. But this is done with certain circumstances usually involving lots of buildings
An ambush on the other hand, which occurs during close combat phase is initiated by a unit moving during the advancing phase into another hex with a unit.
This may be because it's ambush terrain ( woods or a building in ETO, jungles Kunai, or bamboo in PTO), or 1 of the units if not both are concealed
But seeing overrun happens in the movement phase no ambush per the normal governing of the rules
It's all set out in the overrun flowchart. Which apart from the OVA flowchart are pretty much the only 2 flowcharts I will regularly consult
Hope that helps