there will be a trim piece that goes around ur HU, AC, then across, over the steering wheel, ect. Its held in place with clips. Carefully pull this piece off. to make this easier tilt ur steering wheel all the way down. and if a column shifter pull all the way down to 1st gear.
Oncee u have it off, u will beable to see how the stock HU is installed. There will be a couple hex head bolts holding it in. take these out with a socket/ratchet. U should now beable to pull it out. Disconnect the Antenna and wiring harness from the back.
U will need a antenna adapter as u mentioned and a vehicle specific wiring harness. Wont cost more than $15 for both. If ur installing a single Din HU (which is the most common size) u will need a installation kit because ur stock HU and opening is a Din and a Half. U can buy a Din and a Half HU. But most people dont.
1992 Bonnie SE, freshly overhauled, 6000k HID'*, SensaTrac Struts, 32mm Tubular SSEI sway bar
Kenwood HU , JL 1000/1, 8W7'*, 15" CVX'*, 18" DC lvl4