First step would be to lift carpet/floor in trunk looking for coolant. If present, one issue you have will be cracked reservoir tank (very common - even not knowing your MY). If none there, then move on to a quick pressure test on system. This will point you in right direction.
Most voids in a closed loop system (tank, H2O pump, cracked rad, etc.) provide you w/ 'opportunity' to drain and refill system. Don't waste time doing until you find source which allowed air to enter causing boil over. At ~ 6 gal coolant, your H2O added (unless more than a gallon) did little to affect boiling or freeze point. Every pound of pressure on coolant effectively raises its static boiling point ~ 3° thus why w/o it you get boil over...
Good luck
