a little more precise, less telegramatic would help.
Assuming it does what you seem to be saying: to blow both fuses when you press the starter button, it could be the starter relay (#14 on wiring diagram). Unplug that and test.
It could be the stop relay, #19.
It could be a short cicuit between the handlebar switch and the bars. Happens quite often. Remove the switch from the bars and test. If so, wrap the bar with tape and put it back on.
It could be the rectifier #16.
It could be the harness itself.
Has been one or all of these on other bikes.
Some of this stuff has just recently been discussed:
http://www.mzriders.com/viewtopic.php?f=3&t=2390http://home.pacbell.net/adw_msw/mz660_wiring_diagrams.pdfThere is bound to be more; use the
search. That's what it is for.
wirea660