is there such a Harness where i can just use to convert my stock 99 civic si deck onto my 94 civic hb.

 

get a female standard after market harness for your 94 (the old school style harness) and then get urself a male "reverse" harness that copies the factory plug from the 99 civic and match the colors up and plug it in.
like 30 bones total from best buy or get urself a hook up and pay like 5 bucks.