short circuit in the electrical system
 

Well my friend has this 88 GTU and the brake lights do not work.
The battery is also getting drained even tho it's a brand new one.
Any tips on how to fix it?
thanks

he *might* have put on the battery back on backward

there was sparks and the main fuse blew

Lol, I think you just answered your own question...

The only other common occurance causing the main fuse to blow is shorting the alternator charge path cable to the engine during maintenance...

so how would I know which part of the harness to replace?

do a current draw test with a digital meter and start checking which fuse the problem circuit is on.