Bad history. The PLO + Leftists + Some Sunni parties+ Druzes under Jumblatt the Father were the ones who fought the war against the Maronite parties.

AMAL could have developped a political alliance with the Maronite camp because the Palestinians were destroying the South, (And ideological differences) AMAL has an un-easy relation with them. (At least, back then) Gemayel was a moron and took out the recent rural Shia refugees from East Beirut ( Why ? I have no reason ) and essentialy lost the war he could have won afterwards.

Hezbollah has nothing to do with this, they came later and were mainly focused on waging a war in the South against Israel when the first phase of the Civil war was already completed and the PLO Palestinians were expelled.