You have to get rid of (all) the control towers Legion have parked in the mountains to the northeast. But you have to time it with a Legion attack because else they will just switch to apeman/manual vehicles. So wait for Legion to attack with a remotely controlled implosion vehicle, destroy the last control tower before te implosion vehicle is destroyed/explodes and then repair it back up. IIRC it should change to your side the moment you start repairing it.