Recently purchased a '99 disco with Harmon Kardon - previous owner's tape deck stopped working and he wired some sort of AUX input. If you look at the photo of the removed head unit, there's a small black push button type switch on the middle top of the brown circuit board. When depressed and released, the stereo momentarily goes to "SIDE 1" - then "TAPE EJECT" and stays on "TAPE EJECT" - when fully depressed and not released, it doesn't detect a tape and won't allow you to select tape on the face of the unit. When I pulled it open, the switch was taped down into the depressed position. The wires soldered onto the left upper corner go to a standard headphone cable.

The previous owner claims this was working fine - but it took me a few weeks to try it out and I can not figure out why it's not working now. Was hoping someone with some experience with this may be able to help get it working.

 

Hi.
That brown board has to be the electronic part disemboweled from the removed casette unit.
That black pushswitch sensed the presence of an inserted cassette.
I could help you if you can find the schematic diagram for the head unit.
----> Check the cassette 'eject' button on the front panel is not stuck in.

I have done the conversion, but without removing the cassette mechanism, and tapping the 'headphone wire' to a chip on the board.