I just replaced mine the other day because it was leaking and giving me a code 40. It's called an A/T protection switch. There are 2 wires going to it that are secured with a knurled, spin on, lock.
It's very easy to replace and it cost around $60.00 if I remember correctly. It is located on the drivers side of the tranny just forward of the modulator valve. There is only 1 o-ring on it and you'll see that when you remove it.
95 E320 157k miles
00 C230 Special Edition 42k miles