If you have a soldering gun or iron you are one step ahead. All you need is the adapter to fit into old radio spot and the aftermarket adapter harness that plugs into your factory harness.
There are 3 simple power wires, 12v constant (Red wire) , yellow wire 12v switched power (gets signal from the ignition to turn on), and the ground (black wire).
The rest are the speaker wires, two per speaker. The harness adapter is labeled with speaker locations and if they are the positive or negative wire. You just match those up and solder them.
Always use the shrink tube over soldered connections, or at the very least black electrical tape over each wire.
89 GT Hatch: Custom M90 Supercharged w/ Ported Trickflow Trackheat Heads, Stage 1 cam, Streetheat lower / Sheet metal upper, Tweecer RT & PLX Wideband, 75mm TB, 1.6 Roller rockers, 3.73s, MSD 6a, Canton 7qt Pan+ Windage tray, 36LB Injectors, 200 amp Alternator, 75mm Pro-Mas MAF, 190lph fuel pump,Spec Stage 1 Clutch,SouthSide LCA's, Pro 5.0 Shifter, Steeda CC Plates / Springs, FMS Aluminum Driveshaft, New T5 w/ 2.95 first gear, Underdrive pullies, Zirgo 3300Cfm Elec. Fan, AFCO alum. radiator