Volt meter showing discharge

[Guana]

Drove blazer the other night and noticed when putting on the running lights or brake lights the volt meter would peg out negative and eventually battery died.

Brought blazer home, charged battery back up and began to troubleshoot:
>started blazer disconnected neg post and truck died
>noticed post on rear of altinator were burnt
>had alt tested showed good but still replaced
>installed new alt if you rev up engine volt meter shows + if you put the lights on goes neg


Do any of you have any other ideas.......thanks