1990 Cutlass Supreme problem

i have a 90 Olds Cutlass Supreme that is driving me crazy. we had a problem with the fuel pressure regulator, replaced it, the car ran fine. my daughter left the interior light on one night, and it has been messed up ever since. we put on a new battery, and it would just crank over and not even try to start. it was getting fuel and spark, but nothing. replaced the regulator again, now it only runs off of 3 cylinders. it is a 3.1 L V-6, and it only runs off the front 3. the back 3 are dead. have replaced wires, plugs, and everything else i can think of, and it still misses on the back 3. have checked all vacuum lines, and they are all connected. anybody have any ideas, before i sledgehammer this thing?

thanks, everyone.

most likely you have a bad injector. these vin code "t" 3.1 engines used an injection system that batch fired 3 injectors together. One bad one would usually use up all the current so the remaining good two injectors would not fire. you really will reall need to pull the plenum to get at them. You can ohm test them see if there is a stand out amoung the three, but ohm testing is really half them story, as it doesn't test them under load. But more than likely this is the problem.