Directx9 ambient occlusion

I am making a dx9 ao shader.
 
I have the ao greyscale image in a texture, and the rendered scene in the backbuffer.
 
What blendmodes (dx9 fixed function pipeline) would I need to multiply the ao texture against the backbuffer. 
 
something like this : backbuffer=backbuffer*aotexture
 
I currently have the following, but it is not blending with the backbuffer.
 
gRender->SetRenderState(D3DRS_ALPHABLENDENABLE,TRUE);
gRender->SetRenderState(D3DRS_SRCBLEND, D3DBLEND_ZERO);
gRender->SetRenderState(D3DRS_DESTBLEND, D3DBLEND_SRCCOLOR);
gRender->SetTextureStageState( 0, D3DTSS_COLOROP,   D3DTOP_MODULATE );
gRender->SetTextureStageState( 0, D3DTSS_COLORARG1, D3DTA_TEXTURE );
gRender->SetTextureStageState( 0, D3DTSS_COLORARG2, D3DTA_DIFFUSE );
gRender->SetTextureStageState( 0, D3DTSS_ALPHAOP,   D3DTOP_SELECTARG2 );
gRender->SetTextureStageState( 0, D3DTSS_ALPHAARG2, D3DTA_DIFFUSE );

Is it even possible to blend a texture with the existing backbuffer?
 
 
Thanks

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