Best way to connect my old to my stock radio?

So you are going to tap into the rear spark wiring to go to the AMP, then ground the amp in the trunk and run power from the battery to the amp, make sure you use a fuse at the battery. Next the white or blue trigger wire need to go to an accessory that comes on with the key. For this you will need a test light. This guide can help

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

You can use the keyed power wire at the radio as well. Just know there are two power wires going to the radio so make sure you get the right one or your battery will drain down over night. Let us know if you need anything else. Upload an image or short video of any problems you run into so I can help. Ken