I am currently doing the same job.
1. Check #12 fuse for 12 volts.
2. Take out the glove compartment, approx. 7 philips screws, and locate orange relay to the left. Hold it while engaging the over drive (actually underdrive) switch on the shifting lever. You should feel the relay click while engaging and disenganging the swithch. Take out the relay and check for continuity between post 15 and post 86. Check for incoming 12 volts to #15.
3. Get under the car. Make sure it is well supported and safe. The selenoid is on one side of the tranny, but the wire to it goes up and over the tranny to a connector almost directly opposite of the selenoid. Make sure you have the ignition on, the overdive switch off, and check for 12 volts at this connector. This is where I was, when I knew I had to take out the selenoid.
4. Disengage the shifter arm at the tranny to gain access to the metal clip where the wire goes through and take off that clip, too.
5. Now comes the hard part, getting the selenoid off the tranny. You will need to buy, make or find a 12 mm wrench bent in the shape of an "L". There are two bolts which must come off, and all I can say is good luck.