One tip I found for AC... I wrapped exhaust heat tape around the metal AC line going from the evaporator to the condenser (the line that gets cold when AC is on) since it helps that line from absorbing heat from the engine compartment. I got an additional 5-7 degrees cooler output with this setup, and my compressor kicks on less frequently, so I also squeezed a few more drops out of every gallon of gas.
Knowing GM as I do, I wouldn't be surprised at all if those extra wires you see in the trunk are pre-wired from the assembly line for accessories/options you don't have. GM did this to minimize cost and to simplify assembly - one wiring harness for all cars, and most options are simply plug-and-play into the wiring already present). I looked in the trunk of my '87 LeSabre really quick and I couldn't find those wires you describe; however, they may be behind the interior trunk carpeting pieces in my car. Be that as it may, I can offer a few suggestions:
1. Power antenna wire? (Check for +12 at a wire only when the radio is on - of course, this only works with factory stereos and properly-installed aftermarket radios, if the deck supports a power antenna and the wire is connected to the deck inside the dash)
2. Power trunk release? (I don't have this option)
3. Power fuel door release? (Not sure if this generation of LeSabres had this option)
4. Trunk light? I have an orangey-red wire going to my trunk light.
5. 3rd brake light for the LeSabres with the trunk luggage rack option? (Some 3rd brake lights were mounted on the exterior on the luggage rack; cars without the luggage rack option have the high-mount brake light on the rear package shelf)
6. Power lead for factory amplifier? (Totally guessing, I don't even know if the Concert Sound II option had an external amplifier or where it was located - I've seen this on a Mercury Villager, so it'* worth suggesting)
Hope this helps you! Don't worry, there are a few LeSabres of our vintage still out there... I saw a maroon one today that was immaculate. Also, the Park Avenue of similar years have mostly the same parts, in case you need to find parts to restore your LeSabre.