Without even looking, I can tell you what you need to do.
Put a WP before the GETIN for the M113. In this WP init line put "_x assignascargo M113name" foreach units group Somesquadmember where M113name = the name of your M113 and Somesquadmember = the name of some unit in the group who is supposed to board the M113 (usually the leader).
What this does is makes sure that the squad knows that they are supposed to getin the M113 and not the jeep.