Electrical, always a fun one!
 

I have a 2006 ford econoline. When changing out the battery I started to hook the cables up backwards and was interrupted by a large spark. I then correctly hooked the cables to the correct posts. Afterwords I noticed a few things were no longer working....power windows, radio, instrument panel. Thinking they had been blown I checked the fuses under the drivers side, all were good. I then checked the power distribution box under the hood, all relays and fuses appear to be working. Is there somewhere else I could be looking? Thanks in advance for any suggestions.

Welding with an automotive battery is never good. Happens often, however !
Don't know if this will help, but try disconnecting the battery cables off the battery ,and holding the ends together making contact for a few minutes. This should allow the internal charges to dissipate. Then install them again correctly and see if that helps. Sometimes a module gets messed up internally .