Best , cheapest and easiest way is with a cheap 12 volt test light , sort of like a screwdriver with a wire hanging off the handle end , should have an alligator clip on the end of the wire .
These cost around ten bucks for cheap ones , the alligator clip goes onto something metal that's earthed , the screwdriver tip should be pointed and needle sharp , this end you just push into the positive wire at the back of the radio .
Usually a red wire and the earth is usually a black wire .
You need to work out if the radio is wired up through accessories or direct , ie key on acc to work or live all the time .
Test the tester on the battery first to make sure it works and make sure whatever you connect the alligator clip is connected to a good earth , easy if you have a multi meter .
When I say push the probe into the wire at the back of the radio I mean 25mm to 50 mm from the unit , don't push it into the unit where the wire comes out .