i have 2000 grand marquis i want to add trans. in-line filter how do i identify/locate the return line at trans. or radiator thaks for any help ed

Look at the passenger's side of the radiator.
You will see a steel line going into the TOP of the side of the radiator tank.
This is the INLET side, coming FROM the transmission.
The steel line that goes from the BOTTOM side of the tank is the RETURN line that goes back TO the transmission.
Your Grand Marquis transmission will last pretty near forever if you change the INTERNAL trans filter at 30,000-mile intervals.
The correct filter part number for your 2000 Grand Marquis trans is: Motorcraft FT-105 -- it comes with no gasket because the original gasket can be used again and again.
Hopefully, nobody replaced the original gasket with some cheap cork gasket that requires a ton of sealant.
Get the RIGHT filter and gasket, and forget about ATV Sealant FOREVER !!
As for the external, in-line filter -- The correct part number for the in-line filter is: Motorcraft FT-183.
I use genuine Motorcraft parts for a guaranteed best fit and function. Cheap stuff SUCKS !!
You can get good prices on genuine Motorcraft parts at both Amazon.com and RockAuto.com -- and they DELIVER to your door !!