Without re-reading everything, I'm assuming that you are putting a traveler/message center in your Spirit. Hopefully you have the unit from another Spirit or Acclaim since that should be a perfect fit in-place of the cubby hole tray (or what ever it is called

). I believe a LeBaron unit has different physical dimensions, although it is possible that the connector may be the same on the back of the unit.
I did put a combination Message Center/Traveler in my 91 Spirit (Transplanted from an 89 or 90 Spirit with a factory unit). I cut the wiring harness off the donor car so that I had the pigtails and the connector. In fact, I made sure to go way back in the harness so my pigtail had plenty of wire. I only used the Traveler part of the message center since my car did not have the wiring, and extra door switches, etc. As I recall, the Traveler portion must receive a ground wire from the message center connector in order for it to work.
For the data buss, I used two pieces of 22 or 24 gauge hookup wire (like telephone household wire) and twisted the wires together. You should be able to pick up the data buss on a blue connector right above the brake release cable. That is a transmission diagnostic connector for the 4 speed unit. I believe that was wired in to the PCM on almost all cars whether or not they had the 3 speed or 4 speed. Mine is pretty much a base with the 3 speed.
Again, if memory serves me right, you pick up the power and light intensity for the Traveler right off the connector that goes to the radio. Then you pick up the gas tank sender signal from the wiring harness that goes to the instrument console. For me, the job was pretty easy, but I had the instructions from poster JFH. You should be able to find all that old info. There were a lot of EEK enthusiasts doing this mod about 7-10 years ago when the self-serve yards were full of AA body cars. Likewise, a few were doing this on the AJ bodies as well, but the instructions were slightly different (mainly the color of the wires).
You could probably use the tee-tap connectors to tap into the different wires. I simply sliced a bit of the insulation of the the wires that I tapped into and soldered and taped these, but I'm a little old fashion and there was a time I didn't trust those crimp taps.

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Good luck with your project.
To Bob L: Sorry I missed your post about Inst. and Avg MPG calculations. I know for the late 80's to mid 90's (and later) vehicles, the calculation is made from the injector pulse width. With a constant fuel pressure and known nozzle size on the injector(s), the pulse width determines the fuel being used at any instant. The average MPG is a continuous integration process at a high sampling rate and the fuel usage part of the equation is pretty accurate. Of course, the other bit of information is the distance, so to be accurate, OEM tires, correct transmission gearing (OEM), and a properly functioning distance sensor is a must (or simply, the correct pinion factor).