The message center should work since a 98 uses the same VSS signal as the 99. (VSS signal is actually distributed from the ABS module) That is the big difference you need to look for. Well, there are others too...but in this case, you should be fine. If you are really concerned, get your wiring diagram (you will absolutely need a factory wiring diagram) and check all the pinouts off the ECM and make sure they are all there for your message center. They should be.
The EATC should also work. I'm trying to think of a reason it wouldn't and can't come up with anything. You just need to make sure you have the same "color" head unit that the system calls for. You also need to have the ENTIRE system. You can't be piecing those together out of different units. Remember to use an EATC blend door actuator...and in fact, I would take the opportunity to put a new blend door, actuator, and heater core in at this time. You will also need the fan speed controller. Can't use the resistor set up normal climate control uses.
If you pay someone straight time to do this job, it would be cheaper to sell yours and buy one with all this in it. Seriously. You will have a labor bill approaching 2 thousand dollars easy. Remember, the whole dash will need to come out. The plenum assy is different on EATC. In fact, even the evap case under the hood has a different part number for EATC. These are good mods if you do them yourself. If you have to pay someone for their time, forget it.
