Hello i'm new to this forum group and i'm having a heck of a problem with my 2000 Deville.

I had a friend of mine install a new battey into the car but he hooked it up backwards (this was about a week ago) now my car will not do the following : pwr windows,pwr door locks , radio, pwr mirrors,heated seats turn signals/brake lights and some of the guages are not working. i took it to a mechanic who told me the onboard computer was good but thats about it. so i was wondering if anyone had seen this happen before and know how i might go about fixing the problem, nad how much it would cost to fix something like this.

THanks, Jon