Leganza wont heat up

Have a 2000 Leganza that runs cold. Usually that would be a good thing except that in the winter time it doesnt heat up enough to have the transmission shift into high gear. Just drove back from lexington, Ky heading to Owensboro, Ky and the car wouldnt shift into high gear until I got to Elizabethtown, Ky about 50 miles away because it was below 25 degrees outside. Finally stopped and put cardboard in front of the radiator and it helped abit, Any solutions?

Fixed the problem. SOLD the car. Went back to an American made vehicle. Best decision I ever made.