Remove all unused wiring and etc. from the factory harnesses...
or do it the easy, Painless, way
Spaghetti Menders also sells a complete harness setup that is lighter and more efficient... for around $1500, so I'm told. Just tell them what accessories you use and they make you a custom harness.
I'm doing it the hard way, and have the entire interior stripped out of my car, again, going through all of the wiring... should be done in a couple months, lol. It's crazy how many wires are routed through our 'stangs that aren't even being used for anything...
....another thing, remove the heater core and all related components, ducting, etc.
....swap from an airbag to a tilt column if you have, what, a 90-93... I have an 89 tilt for my 93. Quite a few wires routed through the car just for the airbag setup too.
Not a real 'fun' way of lightening your car, but it's a necessary evil (wiring). And you're not going to lose a substantial amount either, obviously. Makes for a sanitary engine compartment and under dash area, that's for sure!