Today's codes are pretty much the same.

Historic
B1552 Keep Alive Memory (KAM) Error
B1911 Generator L-Terminal Problem

Read more at: GM General Motors B Body OBD Trouble Codes
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Generator L-Terminal Problem

Read more at: GM General Motors B Body OBD Trouble Codes
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B1981 Battery Voltage Low
B1983 Device Power Circuit Low
Current
B2750 PASSKey Data Communication Failure
U1016 Loss of Communications with PCM
U1064 Loss of Communications with DIM
U1088 Loss of Communications with SDM
U1040 Loss of Class 2 Communications with ABS
U1152 Loss Of ACM Serial Data Communication
U1255 Class 2 Communication Malfunction (Serial Data Line Malfunction)

I am not sure if it has anything to do with it but the AC blower is bad and one of the connectors on the blower side was black. I ulled the connector and check ground and voltage and they are ok.
The speedometer is accurate until the dashboard start acting up. As soon as the dashbaord goes back to normal, so does the speedometer.

thanks for your help

I do not know if the 99 Deville is similar to the 92 Seville but the left side blinkers and right side blinkers each have a separate fuse. The left side fuse also has the illumination for the dash lights and gauges on it. If it blows you lose all left parking/turn/brake lights & all interior instrument lighting. Before those 2 fuses split off though, it gets fed from the security system. You have security system codes as well. When you say that when you turn on the left blinker and the dash lights get erratic but then normal when you release it, that tells me that something on the left turn signal circuit leg is either shorted when the switch is closed or ungrounded as mentioned above. I don't know specifically what, so you'll need a circuit schematic for your particular model. This will however narrow down your search and give you somewhere to get started.

the schematics I have for the 98 deville seem very similar to the 92 seville. How would you go by finding the short or the bad ground?
Another piece of the puzzle I discovered recently: if I pull the headlights knob one notch back, in manual position, I have no issue whatsoever, everything works perfect and the left turn signal does not affect anything anymore.
I am not strong on electrical circuits but I have a tester.

Thank you

My mechanic finally found it, as EricD suspected, it was a lose ground under the instrument cluster

I installed a new Rockies bypass pro and I'm still having issues I attempted to put in some LED lights instead of the halogens and now I can't get the car to start. Did you ever install but success the pro module bypass?