188.8.131.52.6 discusses modifiers for ATGMs under GP Direct or Indirect Fire. Is this only for leg ATMG units or is there also a modifier for vehicle-mounted ATGMs under direct AP fire such as the M150?
The rule makes no distinction between crew-served ATGMs or those mounted on vehicles. Both are subject to the under fire effects from GP fire.
Vehicles are not subject to under fire effect from AP fire. AP fire either hits or it doesn't (even with no damage results). The modifier reflects the explosive effects of GP fire on controlling an ATGM.