Welcome Aboard!
When the defrost is on, the AC compressor should also run to help reduce humidity in the incoming air. This extra electrical load and mechanical load on the engine can tax the alternator as well as slow the engine, making the alternator even less able to maintain voltage. Your '88 may still have a carburetor, and it may need to have both the curb idle adjusted and AC idle control solenoid checked and adjusted if necessary. The curb idle should be listed on the VECI label under the hood, and the AC idle control solenoid (if equipped) should add 75-100 RPM with the compressor running.