installing an after market deck

[Custom_Celica]

i recently bought an aftermarket deck and went to install it. i ran into a problem with matching a harness to the factory one. I was told that because i have a 91 celica gts that they no longer make harness's for this year any more because it is too old and the harness that is made for my generation celica is too big and only works on 92 and up celicas. i was wonder if i am getting screwed over cause he says the only way to get my factory deck installed is through extensive rewiring of the car due to the amplifier. If anyone can help me, or let me know of a company that makes an amplifier harness specifically to this year

[barkroc2]

system 10 speakers i assume? u should have a factory amp under the seat, and you can buy that harness on ebay