The radio in an 86 should be a becker 754 and there is not user resettable code for it. If you haave the 4 dashes in the screen then it needs to be reset by the mercedes dealer with their reset tool. Usually if you have the 4 dashes there is a any number of problems with the radio #1 a dead lithium battery etc...............If you have any repair questions contact my repair shop @
www.lajollaaudiorepair.com
Scott@lajollaaudio