SuperTexan

Does anyone know how to use a jumper wire under the dash to read a code on a 93? I had a little Cavalier I was able to do that on, then count the flashes of the check engine light. Previous owner of this truck has mangled up the wires under there somehow installing a trailer brake switch. There is no plug in anymore to read codes....My truck has been doing this thing where it wants to try and die when I come to a stop. What it feels like, is like the transmission is not down shifting. Then that episode is followed by the check engine. It will run like crap at idle, and "stink"...not rotten eggs, but just a lot of exhaust seems to be in the air. Sometimes it will die, and is hell to restart. This has been going on for almost 6 months, but is getting worse. But I dont know where to start.