TomGeorge:
Hi,
I'm sure a long way back in this thread I asked why you aren't using dedicated Automotive High Side Switches.
They are purpose built for your application.Tom...
My initial take on them was that they are a bit slow for PWM. This appeared to have been corroborated by other contributors.
I would be ecstatic if you find me something like that that can manage 10A per channel at PWM rates north of 980Hz! 
Cheers!
Dirk