1999 Plymouth Breeze Car goes into limp mode

You could have a bad Power train Control Module, (PCM), Body Control Module, (BCM) which is also referred to as the body control computer which you will see in the attached information, or the Programmable Communication Interface, (PCI) bus. In the attached information you will see that the BCM controls the VIC, Immobilizer or anti-theft system and the instrument cluster. The proper signal depends on the PCI. Another possibility is the CCD, see attached.
These modules or the PCI bus could be causing the vehicle to go into limp mode and then it explains the electrical light if the PCI bus has a transmission issue. Also the increased fuel consumption can be the BCM or PCM getting the wrong signal or they might be failing. It is hard to diagnose electrical problems as you have to start at one point in the circuit and trace it and test components on its path. The circuit is very seldom a 1 wire path but a parallel path.
The best place to start is to have the trouble codes pulled. Advance Auto or Auto Zone will do this for free.

I forgot to attach the information sheets that will help you understand what controls different components and how one bad components can affect all the others.

That is a good idea. If you are looking for a reputable repair shop near you, use the search on this site and it will give you repair facilities near you in the text and banner ads. You can also look for shops that rebuild PCMs. You can use the search feature for that too.

Electrical problems are the hardest to diagnose. Especially if your not getting any trouble codes. My best suggestion is to get a reputable shop to chase down all the possible circuits to make sure they are not shorting out or there is a broken wire.
I had this happen to my fiancés 96 Mustang GT. I was so stumped that I took it to the only place that I let touch her car. You can find a reputable service shop near you by searching our site for repair facility or shop and banner and text ads for shops near you will come up on the page.
That is my best suggestion. I would send you the wiring diagrams but I would have to send you so many it would exceed our gate window and you would have to trace the circuits yourself which you have done to a great extent already. So, I suggest taking it to a shop and ask if they have someone who is good at electrical diagnosis and talk to that person and describe your problem. See if they have any confidence in being able to trace the problem down. "We'll see what we can do" is not as good as " I am sure we can fix it" as an example.