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Drivers side of transmission.
On the valve body, inside the transmission.
The transmission dipstick is located on the passenger side of the engine compartment, adjacent to the evaporator case.
Remove the transmission oil pan
A 2003 Dodge diesel manual transmission will work in 1990 Dodge diesel manual and bolt to the 1990's transfer case with little fabrication.
if they have the same transmission then most likely but the 1990 is prob fuel injected and the 85 is not
The oil drain is located behind the left sidecover. There you will see a hose with a metal plug in the end held in place by a clamp. Loosen the clamp, remove the plug and oil will drain from the tank.
Inside the trans pan.
The 1990 Chevrolet Corsica TCC transmission solenoid is located on the top of the transmission. The solenoid will be near the back of the transmission.
If you are referring to an automatic transmission drain plug there isn't one.
With a manual transmission, it is on the passenger side of the transmission case. There are two 3/8 square plugs. One is the drain and one is the fill. With an automatic, there is no drain. You have to remove the pan.
This vehicle do not have a transmission filter. All you have to do is drain the trans. fluid and refill.