I used the POR-15 tank coat - to BAD results. I even doubled the recommended cleaning solution procedure.
Skip it.
Be sure the radio is 12 if that's what your car is before you try it. It will be marked. If it's a 6 volt radio, you'll fry it on a 12 volt car.
Don't be discouraged if it doesn't work. Few of them do 40 years later. be sure to also have a known good speaker hooked up, and an antenna fully extended. They can fry if there's no load (speaker) and being AM band, need an antenna to pick up anything.