This has been a problem for us since ODS went FF. My oldest is almost 7yo and youngest 3.5yo. It didn't get any better until we lost our RN and I FF the 3.5yo b/c he maxed out the borrowed seat RF. So, I *feel* your pain and am still debating RF vs FF the 3.5yo, so on the positive side, it gets better eventually!
And, ROFL about that duct tape comment, I've seriously thought about that too! I've also threatened (joking of course) to put the 3yo in the trunk (even better I drive a hatch back we don't have a trunk), but it really smartens him up for some reason! LOL I suggest not threatening the roof, for some reason that made him cry, perhaps b/c he knew what it was! LOL And, again, joking, how would I secure the car seat? LOL
And, I've done the radio trick, open/close every window in the car, yelling, stopping the car, getting out of the car and I'm sure a dozen more. Taking the 3yo out of the car and closing the doors worked once, it was a few weeks after our crash and I might have just lost it. I couldn't stand the screaming and still had the after crash jitters...so took him out and told him he was going to have to walk home. Think it worked b/c I was *not* kidding and we were only a couple blocks from home, so certainly could walk home. He'd kicked ODS in the face...
tam