Your 88, like mine, has the connector to the factory stereo hidden just to the left of the box that encloses the stereo in the dash.
To pull the stereo, remove the knobs, then using a pair of small screwdrivers, pull the two spring latches inward- you can see them just outside of the knob holes. When released, swear a few times and the stereo will eventually come out. It's tricky tokeep the latches pulled in.
Anyway the wiring harness ends in a 9-pin connector that then meets another 9-pin connector and goes to the amplifier. Under the steering column, there's a big 20W amp. That's where ALL the speaker outputs go- through a circular plug on the back of the stereo, and over to the amp- then the amp powers all 4 speakers.
I tied all the new stereo outputs into the speaker wires leaving the amp- and cut the amp completely out. The stereo plug contains all the power wiring to the new unit. An extra plug has just the rear speaker wires in it. It's all in the space behind the ignition switch. I removed the whole lower dash cover to get at it all.
You will need a faceplate for the new stereo as well. This and the wiring harness are available in a kit for about $15.
Good luck!
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Rob Bareiss, New London CT ::: '87 244DL/M47- 220K, 87 244DL- 230K, 88 744GLE- 198K, 91 244 180K, 88 244GL 145K