You cannot powder coat plastic! It will melt way below the temperature required for the powder to melt. Most plastic melts @150°-250°. Powder melts @450°.
How you electro-plate plastic: Apply conductive primer. Electroplate same as you would metal, copper-nickel-chromium. That's how Mopar did it. Then all your paint, decals, whatever, get applied.
On Later cars (don't know when they switched, maybe mid-70s) the chrome trim was "printed" on using heat and a metal foil directly onto the plastic. No where near as durable as the real plating, but obviously much cheaper.
