I think that one other option would be to convert your tape collection to MP3 and replace the stereo with something modern...
One example...(there are tons of products in this space)
http://www.amazon.ca/Cassette-Conver.../dp/B005PU2SMS
Also keep in mind that back then they often changed the harnesses from year to year / model to model as CD / CD changers displaced cassette players in the late 80's / early 90's.