That's the amplifier for premium sound. There's usually a harness that goes between amp and radio. Pull it out and unplug the amp. Then you can use those plugs for the radio. You don't want to run the oem amp after an aftermarket head unit, which has its own internal amp that is better than 30 year old tech,
Toss amp on eBay.
You'll lose door speakers, but you'll want to run your own wires for those anyway and run doors and rears with your head unit. Dash speakers can be used with good low frequency blocker