260z Electrics burnout.

[1200 Koozer]

Hi guys other day i started up the 260z and almost instantly headunit stopped working and smoke started billowing from under bonnet. What has happened is the ground wire from the alternator has melted all insulation off and traced up the loom to where it bolts onto the chassis. There is also another ground wire that runs from the block and splices into it. Just recently had diodes ans bearings changed over in alternator after it blew but since then has been fine until now. Any ideas what could cause it. What confuses me is why it has pulled soo much current through ground wire.