Just want to confirm this. It's past mid-night and my brain is not working.
LEt $\Phi(x,y)$ and $\phi(x,y)$ be the CDF and PDF of bi-variate normally distributed variables.
Then
$\int_{b_{i-1}}^{b_i} \int_{a_{i-1}}^{a_i} \phi(x,y) dx dy = \Phi(a_i,b_i)-\Phi(a_{i-1},b_i)-\Phi(a_i,b_{i-1})+\Phi(a_{i-1},b_{i-1}) $