Power locks

[plumber2311]

The power locks on my 97 E-350 have a mind of their own. The front doors do what they want when they want, un lock when locking and vice versa or won't respond at all. My back door doesn't work at all, but the side door works as it should from either switch or the fob. Where do I start?

By the way, what is the electrical box mounted on the wall behind the drivers seat kind of on top of the fuel door?