Just a thought...... Try getting your car to operating temp. and turning on the defroster...... if your windshield fogs up, you may have a heater core problem, still a quart every 50 miles sounds a bit high for a heater core.

Also you may want to check your floor carpeting for signs of dampness, they can hold a gallon or so.

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1993 190E 2.6 135k
1989 Ford Thunderbird SC 5 Spd 79K