I don't know what you are using as an amp but if it is the apline those things can get damn hot in the dash so drawing in air from near the amp circuits in it might make things worse as the heatsink for the amp circuits is normally the case.

I had an alpine that used to get very hot indeed too hot to touch occasionally when running hard even got the high temp trip to activate on it once and i live in scotland air con ain't very popular here.

It might be better to blow out the top holes where the hot air will rise to anyway, sucking air in through the side vents. makes more sense in a heat flow type of way.

You will probably have to cut some holes in the cage/sledge but they are overly beefy any way so that won't harm it. But be warned stainless can be pain to deal with at times.

I don't know how much room you got but you might be able to fit a fan in the dash to help circulation.