Try changing the computer again. They are 20 some years old now and sometimes the little blue capacitors inside will leak electrolytes and cause problems. I had to replace all three in mine because they had leaked and corroded one leg completely thru. If things work with a known working computer, that may be your problem. The caps are cheap, $1-2 each and some careful cleanup and soldering will fix the problem. Of course, he could have done some welding on the car and completely fried the computer too.