2010 GMC Sierra: I need to get into the MBEC under the dash to tap off the 10A battery feed. I have the GM P/N 20791502 connector but I'll be darned if I can't get the cover off the blasted MBEC box. There's a small black button or lock on it and I've tried pulling, pushing, & turning it. Anyone know how to get this bugger off ??

Thanks,
Fred