I don't have a radiator fan to test with , but , since I work around broken computers
I have access to the 120mm x 120mm computer fans.
I've had up to 12 of these hooked up in parallel. The breeze was impressive.
I can say that you'll want to have or build a PWM to control the power. I always had to start them at 12Vdc, then PWM it down to 9Vdc.
This keeps them running nicely and doesn't suck up all the power from the 12 power source. Plus the noise is lower.
I made sure to match the current draw as closely as possible.
I may have a pic up here somewhere.
Hope this helps
Bruce S