Behind the engine on the passenger side, should have a yellow loop handle on it.
Many times they are also found in the back-right side of the engine compartment.
The Owners Manual should have pretty good instructions on this - finding one that old might be a problem.
On a 2001 Mercury Sable : The automatic transaxle ( transmission ) fluid level dipstick is near the firewall , in the area of the brake fluid reservoir , in the engine compartment
The automatic transmission ( transaxle ) fluid dipstick is near the firewall where the transmission is bolted to the rear of the engine
On a 1999 Mercury Sable : The transaxle ( transmission ) is located in the engine compartment , bolted to the engine
Right where the transmission dipstick goes. You will need a funnel
Inside the transmission
On a 2000 Mercury Sable : The automatic transaxle ( transmission ) fluid level dipstick would be near the firewall just to the drivers side of the engine ( where the transaxle is bolted to the engine )
On the transmission.
According to the 1999 Mercury Sable Owner Guide : ( Motorcraft MERCON-V automatic transmission fluid ) ( to confirm the type used , it should be engraved in the automatic transmission fluid dipstick )
I believe the type used is engraved into the transmission ( transaxle ) fluid level dipstick
I know on the 3.8 liter V-6, the tranny dipstick is located on the drivers side at the fire wall next to the brake fluid reservoir. If it is like my sable some of the wiring harness obscures the view. So look carefully.
In a 1998 Mercury Sable : The transaxle is the transmission on your front wheel drive Mercury Sable it is located in the engine compartment , on the drivers side , bolted to the engine
On a 1996 Mercury Sable : The automatic transaxle ( transmission ) fluid level dipstick is near the firewall and the brake fluid reservoir ( looking from the FRONT of the vehicle it is to the left of the reservoir )