I also have a 90 Accord. I would highly recommend getting a Haynes manual for doing your own work. They cost around $15-$20, and cover all that stuff like speaker replacement, wiring diagrams, etc. As far as the wire hanging down, if you have a simple multimeter that can measure voltage, you can see if this is the ignition power wire by doing this: measure voltage by placing the red (positive) lead of the multimeter on the wire, and the black (neg) lead on any ground part of the car (this would be any metal part such as a screw, etc. that makes a physical connection with the body). If this is your ignition wire, it should read 0 volts with the key in off position and around 12 volts if you turn the key to the accessory position. Good luck and f*ck the bastards that broke into your car - I also had this happen once.