1992 500SL - no dashboard lights instrument gauges working after recharging weak battery

working on this dead beast since taking it out of storage 6 months ago, I finally got it running really good like it did for the previous 3 years with no problems after replacing distributors, rotors and the coil cables to the distros and the crankshaft sensor, so then it idled fine at 600 but then a slight loping between 700 & 900 when it goes closed loop, but the cam seal behind the distro had deteriorated after 22 years and was leaking rusty condensation from the decomposed seal spring, and oil droplets into the distro.

but there always has been this one issue; something drains the battery in 2 days time if the ground is left on the battery -- so after taking the passenger side distro parts off for several days and bought new cam seals and some seal pulling tools...
while working on the seals, I forgot to disconnect the battery for 3-4 days so after I got everything back together the battery would not turn over the engine but all dashboard lights dimly lit and the instrumentation worked ..

so I shut off the car took out the key from the ignition switch and charged the battery for several hours with a 10 amp output charger, while I changed out the other 22 year old cam seal - it took almost 3 hours to get the old seal out trying not to score/scratch the cam or seal opening, which wasnt easy, and it took several tries to get the new seal to go in flat. .. finally I put the distributor assmbly all back together ... and was ready to turn it over again and take the car for a spin, dor the fist time on over a year.

So, I disconnected the battery charger put in the key and she started right up like before BUT NOW all the dashboard instrument lights were flashing on and off weakly, they were all strobing very fast at the same time and no instrument gauges were working.
so I shut it off, and tried it again, same thing, I revved the engine to see if it was affecting the engine and noticed it wouldnt rev up beyond about 4,000 like the timing was going off ..

so I thought to disconnect the battery to do a reset -- I left the battery ground off for about 5 minutes , then put the ground back on and started up the car's engine -- now no instrument lights or gauges worked, the engine started right up cold but still had the higher rpm out of time, timing limit ..

I quickly checked the battery while the car was running with a DOVM and the alternator was charging the battery at 13.7V - when the battery was resting right after running it was 12.37V .. I took off the ground to the battery..

I'm figuring the engine is running open loop but didnt want to risk leaving it run any longer in that condition .. with no instrumentation lights or gauges working.

So, I checked the over voltage protection fuses and they were good, then check all the other fuses under the hood on the drivers side just ahead of the driver and they were all good, checked the trunk fuses on the passenger side just left of the battery and they were all good...

does anyone one know what happened ?
did charging the battery with the cars cables on the battery, with the ignition switch off take something out ?
mind you there is already something heavily draining the battery in 2 days time if the ground is left on the battery, but have never been able to find out whats doing it .. which I have been able to live with in the past, because I dont drive the car often, only for pleasure on weekends or to car shows.

But now all the instrument gauges and dashboard lights are not working -- I'm at a lost as what to do next .. and I'm afraid to drive it anywhere now in this new electrical faulty condition, with no dash lights or instrument gauges working.

There is a transistor in the instrument cluster (BC640) that is quite often damaged by a boost or charge of a discharged battery.

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