Cadillac Repair: Crazy Electric dash problem, fuel gauge, climate control

Question
I have just got a 1989 deville.  The check engine light is staying on.  The whole dash will start blinking & then you can not read the fuel gauge or turn off the heat.  We took it to someone who when he tightened the battery post.  The Car reset it's self.  Everything was working , you could read everything.  My husband put the car in park.  Again the dash started blinking..it reset it's self right back to not being able to read the gas & not turning off the heat.  
We are wondering is there a code we need to reset?

Answer
First we need to see what code is stored in the computers.

perform this test:

1. Turn the ignition switch to the ON position. Do NOT try and start the engine.

2. Simultaneously push and hold the OFF and WARMER (^) buttons on your climate control panel for 3 seconds and release them

3. Tell me everything you see on the fuel data display as if you were video taping it.

4. Turn ignition key OFF.