I bought a 90 LX that did not have a radio. The amp is there.
I want to use the amp. Where can I find a wiring harness so that when
I install the radio and speakers, It will work?

 

Do you mean it has a factory amp? If so I promise that you do not want to use it. They used cheap parts and had them gained up so high that they were just noise makers. I always recommend bypassing them and thankfully it is easy to do. There should be two wire bundles going to the amp. Follow one up and it will go up behind the glove box and have a pair of hrn's that can be disconnected. Disconnect them and pull the male side back up to the radio(There is enough slack) and get a standard for 70-1770 hrns set and use THAT hrns as your speaker hrns, and you have now bypassed the amp. That hrns goes straight to the speakers.

The hrns you are looking for is the exact same one that I point to on page 2 of my cardomian page. It is an 8 pin hrns that looks like the grey 8 pin power hrns that is behind the radio.