I have a 2000 tarus,when i drive locally,the temp stays okay. when i get on the highway for about 45 minutes the temp start going up , so far it have not reached hot yet. if i am driving locally with the air on for about 25 or 30 minutes the temp start going up too,

then you have a possible restriction in the cooling system if it has overheated since the thermostat change id change it again cause they are temp sensitive /possibly the radiator is stopped up a little/ enough that it wont over heat around town but on the hyway it cant flow like it should or when you had the thermostat replaced they actually didnt put one back in this would allow the radiator fluid to flow to quickly through the radiator and not cool down then you have a possible restriction in the cooling system if it has overheated since the thermostat change id change it again cause they are temp sensitive /possibly the radiator is stopped up a little/ enough that it wont over heat around town but on the hyway it cant flow like it should or when you had the thermostat replaced they actually didnt put one back in this would allow the radiator fluid to flow to quickly through the radiator and not cool down

see my comment below see my comment below