The OD relay, as others remarked, is easy to get at (behind the vents, accessible after pulling out the glove box -- really easy.

But... (I just found this out myself) the O2 sensor in the '89-on models is hard to reach.
Admittedly, if you had an earlier model, it's located inserted in the exhaust manifold by the engine. That's what I think the others were assuming when they told you how easy it was (if you have the tool, a special socket for the sensor).
But for your model year, 1989 (and 1990), you'll find it inserted in the exhaust pipe just a few inches ahead of the catalytic converter. That might be too far back (i.e., in the middle of the car) for you to reach unless you put the whole car up on four jackstands -- a ramp (lifting only the front) may not give you enough room, which is what I found for my car.
[For 1991-93 cars, the O2 sensor is not in the pipe just ahead of the catalytic converter -- it's inserted directly into the cat.!]

Good luck.

Also, the reason for not soldering is simply that it makes the joint prone to breaking from vibration (it stiffens the wire joint, and makes it fatigue-prone).