Low beams don't turn on except when I turn on high beams

Where are you located . This actually has bearing on my answer as Canadian and European Altima are wired differently than American ones. Canadian ones have a DTRL module (fsm control modual)that screws up and replacing that usually fixes the problem . If its US made then the switch is likely the problem . You may want to check the bulb sockets for corrosion as this sometimes can cause the same problem. However if your Altimas like the ones I see around here just run with the high beams as there not very bright .Most people wont notice ;-) Hope this helps.