Options to hard wire USB for power.

[jchicago]

The PO cut the dash of my 66 C20 and installed a Radio Shack CD player. I personally can't remember the last time I listened to a CD, so as soon as it warms up, I'm pulling the radio and fabricating a delete plate to hold me over until I work on the interior. I want to install a few USB ports on the delete plate for electronics and have one cable tucked in the ashtray to power my smartphone. The plan is to break open a few cigarette lighter chargers and hardwire them in, but all the instructions I find are to wire it into the existing wires, and I want to make sure that the chargers power off when the ignition is off.

Where is the best place to tap into some power for them? Thinking the radio power that won't be any use anymore might work?

Thanks for any suggestions in advance,
Jason