If you put the transmission in 3rd, then it will not shift higher than gear 3. I never heard anything about it going into sport mode, I could be mistaken. I don't know about the '95 tranny, but on the 722.6 it holds the gear right up to max engine speed at WOT, so there is no need for this. Where does the tranny shift? If the tranny shifts at red line, then you are not really going to gain anything from doing this.
Probably need a new shift stalk for the high beams, not sure about that, I personally would leave it if the constant on still works.
No warm start could also be a problem with the fuel accumulator.
The wiring harness is simply the electrical wires that are under the hood. There is a large mass of wires that run through the engine compartment and branch off into the different accessories. Imagine it as the central nervous system of your car. If it is going bad, the insulation on it will crack exposing bare wires and causing short circuits. Just look for a large bundle of wires covered with a black fabric type of insulation, and the wires that run out of it in various locations. Then look at the wires to see if there is any crack in the insulation and any bare wire is visible.