has anyone replaced the factory in a lexus 400 ls 94 . need wiring for the sub amp to work with aftermarket radio

Never changed the deck out it one. If you need amp wires, just buy an amp wire kit. You have the right dash kit for your car? If you have never done car audio before I recomend you take it someplace.

Wonder if I did the same thing for my Gs300

chances are, you have factory installed amps, posibly more than 1, do you have a nakamichi sys, in your lex?(if so, don,t bother replacing, just add a sub) the amps don,t look like aftermarkets ones, sometimes, they hidden, see if that is the case first, second, before you do anything(cutting wires, drilling) try to get wiring diagram, it will help you to avoid many mistakes, also never cut or test probe thick yellow wires under the dash(air bags), you might blow them, i,ve seen that happen. the buttom line is anything can be done, as long as you know what you doing. sometimes it is much cheaper to get it done at a pro shop, than pay to fix your own mistakes.