installed battery backward engine not firing what can i do to fix this
on 1999 Buick LeSabre

A friend said to try turning ignition switch 3 times then try starting vehicle. I tried that and it did not work.

Asked by for the 1999 Buick LeSabre
Assuming the battery is now installed properly , replace blown fuse(s) , main(maybe 100amp) and possibly others. Follow pos. battery cable to underhood fusebox(may be labeled 'power distibution') main should be closest to cable hook-up at box.