my 2003 mercury sable wagon transmission warning light goes on after driving for 20 miles

Follow Tom's advice.

I have a different year - 2001 Mercury Sable Wagon, I'm told by my mechanic that these cars traditionally have transmission fluid issues. He suggested to carry a bottle in my car, so I can regularly check the level, and to keep regular eye on it I think because it just leaks or runs low. He said not to worry too much about it. Also ABS connection is shot due to the age of the car, my brake light stays on all the time now (I have only 52k miles on mine - work from home). It may be good to flush the transmission fluid too, but it is a little pricey.