If I remeber correctly, when I tried the same thing with a "conversion" harness, nothing worked. What I had to do was fabricate my own harness. Basicly one of the connections to the amp are the connections to the speakers. You'll want to interface that to the speaker connections on your head unit. Or at least, that's what I did. I had a spare stock amp lying around, so I opened it up to figure out what connections went where. From there, I cut the connector out of the amp to use the connector on the adapter that I made. Then I ohmed out all the speaker connectors to find out where they went into the stock harness that connects to the amp. Then I matched the amp connector that I cut off with the outputs on the back of the head unit. Viola! Everything seemed to work. You might try searching for a "Speaker Polarity" post that may have the colors and stuff for a '91, it may be similar if not the same thing for an '87.