garden hose and from the engine side of the radiator, squirt through the radiator honeycomb to flush out the debris.
Some caution is in order with this technique. I’ve had problems on my truck using the hose-spray to clean the radiator where the force of the water spray bends the little fins over at a 90 angle and makes the air flow through the radiator problem worse rather than better. Suggest to try it on a small spot in the corner of the radiator first, then look carefully to see if the fins are getting bent.