ya, my dad's 73 240z has the same problem. from what i was told, that is a common problem in z's. i was told that early z car's fuse boxes would build up corrosion on the back side of the fuse box and build up heat thus causing the plastic around the connectors to melt. it doesn't affect the connection, just he box itself.

Matt (AKA BenzGuy)
73 240Z
72 280SEL 4.5(faster than most cars, including most z's)
83 240D

 

mine has a melt mark by the A/C fuse.

 

I just remebered a tech link about fusebox's. Here's the link hope it helps

http://www.zparts.com/zptech/article...ge/fusedam.htm

zya
Yuichi
ZDriver.com
StrictlyZ.com