I dunno but deadening that area may help... a little bit.

If you're dead-set on keeping them in the stock location, you could try and point them towards a desired point in the cabin. Know that this will require making a MDF baffle... fiberglassing, bondo-ing & finishing. (While doing this you could install your tweeters closer to your mids... if you have tweets.)
Additionally, most speakers like to operate efficiently w/in a specific volume/airspace. (Remember that sub box in that back!?) This applies to mids also. To find this out you need to obtain the speakers' T/S parameters from the actual spec sheet. In doing so, you also may create a weather cover baffle.

An alternative to this could involve equalization by a good EQ (PPI comes to my mind... :drool: )