Use car radiators for solar collectors. Any advice?

I have salvaged 2 car radiators, they aren't leaky. I want to use them for active closed loop solar collectors. Any advice? My current plan is to put them on the roof (in a hot box), then plumb them into an old water heater in the attic. My hope is they can recirculate glycol in a closed loop, convection only, but I may need to install a thermostat and pump. Anyone have experience in this? I want to dump the heat to my 2 water heaters in the basement (they are plumbed in tandem, so I can use the 1st tank for solar boost and 2nd tank for final temperature control). Heat exchangers: either the Butler Solar Wand, or Brazed Plate Heat Exchanger (single or parallel). Differential Temperature Controller from www.jc-solarhomes.com. Any advice or suggestions hugely welcome. thanks in advance, Chris

Car radiators should work fine, make sure they're clean and painted black, that's about all I can think of, I don't know how much of an improvement they'd be since they're designed to have air flow through while transferring heat but I'd imagine they'd be as effective as the standard several thin pipes painted black in a hot box...
Yeah - if you somehow circulate the air thru a larger-than-the-radiators hotbox on the roof, then you can collect with a larger area, heat the air, circulate the hot air thru the radiators (in addition to directly solar heating the rads) you could get some extra oomph - just make sure the circulating fans are pv powered :D