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First of all, heat from the radiator is being rejected at ~90C or so. The ideal heat engine will make squat from that, so a better place to be looking is perhaps the exhaust.
The problem with Peltier units is that they have absolutely horrendous efficiency, and most of the ones being produced nowadays are meant for lower temperatures and thus cannot take advantage of the high temperatures found in automotive exhaust (I don't know what the typical temperatures look like, but it gets as hot as 900C at the catalytic converter, and I remember seeing a chart that showed 450C as the lowest temperature). You can certainly buy a bunch of them right now and rig them up to your exhaust, there's a company that someone here gave a link to a while ago in another thread that wants to make a commercial plug and play automotive exhaust heat regeneration unit. They happen to also sell 300C TEGs (which are the best you can get these days apparently) for 100 dollars each, which put out 15W maximum.
So you start to see the problem, to power ignition, lights, ECU, etc. you'll probably need 1000+ dollars for the thermoelectric devices alone (a DIY heat exchanger and cooling system might not be that hard though). Meanwhile, the BMW turbosteamer is I think well under 50kg, and is producing up to 10kW. So that's kinda what TEGs are up against.
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