for a 98 wrangler wire harness should look close to this, though the below harness is what your plugs will go into when hooking up a aftermarket radio
off that black factory plug is where your should find your rear speaker wires.
**factory wire colors should be**
Driver Rear (+) = Brown/Yellow
Driver Rear (-) = Brown/Blue
Passenger Rear (+) = Dark Blue/White
Passenger Rear (-) = Bark Blue/Pink
Sometimes they will change speaker wire colors so be careful. i don't always trust books etc.
generally Gray plus has power, dimmer and 2 of 4 front speaker wires on it & the black plug is generally all rear speaker and the other 2 front soeaker wires.