1990 Cadillac Deville fuel data monitor and other dash warni

Car was stored for couple of years. Subjected to hot weather, sand, and heat. Has that digital fuel monitor. Car needed fuel pump which I did then it ran but fuel data monitor only shows a " d " where the remaining gallons should read. The other part that should read average mpg, miles remaining ect. Is blank. Tried cleaning every fuse and contact I could find, no good. Then just for a short moment or two the monitor reacted showing 2 or 3 gallons remain and the other side showed 0.00 then started displaying all kinds of senseless symbols on, off, just freaking out then when I turned off then restarted went back to " d " and has never changed since. Check for power in and out of sending unit at tank seems to be present. Then I tried just for trying replaced the whole monitor unit with one I got from junk yard, still same thing also the "service vehicle light is on solid, the "service engine soon" light comes on after a few seconds, the brake light is on constantly and the inflatable restraint light comes on, blinks, goes off then back on again blinks again, I don't know--the car generally is immaculate but like I said sat for a very long time. Anyway, first and most important, whats the trip with this damn gas gauge? Then how can I reset the "service vehicle soon" and "service engine soon" light or what? All the windows and door locks didn't work so I took every door apart cleaned out sand and crap from every switch and lubricated regulators, track and whatever and got all windows and power door locks working they are slow, but working. Could the problem be with the ECM or is there a specific control mondule for the fuel monitor? What can I do to get this fuel gauge to work? And get all these other dash indicators resolved or reset or whatever===PLEASE HELP---thank you--lou---jloumatt@cox. Net