On my 89 (your 88 should be the same)...... there were 2 different harness's up under the dash I had to use. One puts power to the newer stereos but I could get no sound from that harness; my buddy tells me that 's because of the way the newer stereos are wired inside. I had to splice into my second harness for my sound.
You do that by going to a proper stereo outlet. They carry a kit for your specific year (88), the wires are color coded in the kit to match the colored wires in the other harness that I mentioned (there are approx 5 or 6 different colored wires; you just splice them together.
Use proper wire connectors and crimp them, don't do a hack job with tape. Your correct ... on your 88 the amp has to go like you mentioned because the newer stereos have the pre amp built in.