I am trying to purchase a cat-back exhast kit from a web site called www.overboost.com and they have some but they are for 1989 skyline GTRs will that same kit fit on my gts-t?

You'll have to modify it a bit.

I also have a question about putting in speakers and such, if i were to get a motorized LCD and dvd player and i wanted to put it in my skyline like this one

http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&category=67772&item=5712216659&rd=1#ebayphotohosting

how would i get a skyline wireing harness so get power to it? or would it just be a nissan adapter?

you can purchase wiring loom adaptors to fit the factory loom.

even for a skyline? would i have to get one from the maker of the LCD and DVD player or from nissan? and is it a skyline adapter or a nisssan one?